The Ophidia | The Classification and the Osteology of the Reptilia (2024)

This order of Reptiles has been dividedas follows:

1. The palatine bones widely separated, and their long axes longitudinal; a transverse bone; the pterygoids united with the quadrate bones.
1. None of the maxillary teeth grooved or canalieulated.

1. Aglyphodontia.

2. Some of the posterior maxillary teeth grooved.

2. Opisthoglyphia.

3. Grooved anterior maxillary teeth succeeded by solid teeth.
   

3. Proteroglyphia,

4. Maxillary teeth few, canaliculated, and fanglike.
   

4. Solenoglyphia.

2. The palatine bones meet, or nearly meet, in the base of the skull, andtheir long axes are transverse; no transverse bone; the pterygoidsare not connected with the quadrate bone.
   

5. Typhlopidae.

All the Snakes possess a scaly epidermic investment, whichit. usually shed in one piece, and reproduced at definite intervals.As a general rule these scales are flat, and overlap oneanother; but sometimes, as in Acrochordus, they becomemore tubercle-like, and do not overlap. In the Rattlesnakes(Crotalus) the body is terminated by several loosely-conjoinedrings of horny matter, which consist of the modified epidermisof the end of the tail.

The derm does not become ossified in the Ophidia.

The number of the vertebrae in the Snakes is always considerable,and in some cases becomes very great, amountingto more than four hundred in some of the large Pythons. Thespinal column is divisible only into caudal and precaudal regions,as there is no sacrum, nor any distinction between cervical,dorsal, and lumbar vertebrae. The atlas and the odontoidvertebra are similar to those of the Lizards, and the atlasis the only precaudal vertebra which is devoid of ribs. Thecentra have nearly hemispherical articular surfaces, and thusdiffer from those of ordinary Lacertilia, while the superaddedarticular processes found only in certain Lizards attain a greatdevelopment in the Snakes. The zygapophyses are broadand flattened, and the outer surfaces of the anterior pair arecommonly prolonged into a process. The anterior surface ofthe arch above the neural canal is produced into a strongwedge-shaped zygosphene, which fits into a correspondingzygantrum of the next preceding vertebra; and, on the posteriorsurface of the arch, there is a zygantrum for the zygo-sphene of the next preceding vertebra. (Fig. 71.)

The transverse processes are short and tubercle-like, andthe heads of the ribs which articulate with them are simple.Each rib usually gives off a short upward process at a littledistance from its head; it is curved, usually hollow, and terminates,inferiorly, in a cartilage which is always free, notrace of a sternum existing. Strong descending processes aregiven off from the undersides of many of the presacral vertebrae. In the caudal region, elongated transverse processestake the place of the ribs. Chevron-bones, like those of the Lacertilia, do not exist, but the caudal vertebrae possess bifurcateddescending processes, which bear similar relations tothe caudal vessels.

The skull differs from the ordinary Lacertilian cranium inthe following points:

1. That vertical elevation and lateral compression of thepresphenoidal region, which give rise to the interorbital septum,are wanting; the floor of the cranium being nearly flat,and the vertical height of its cavity diminishing gradually infront, so that it remains spacious between the eyes, and in thefrontal region generally. The periotic region is not producedinto parotic processes.
2. The boundary-walls of the front half of the cranial cavityare as well ossified as those of its posterior moiety, and thebones which constitute the brain-case are firmly united together.


3. On the other hand, the nasal segment is less completelyossified, and may be movable. The premaxillae are usuallyrepresented by a single small bone, which very rarely bearsteeth. It is connected with the maxillae only by fibrous tissue.


4. The palatine bones never unite directly with the vomer,or with the base of the skull, but they are usually connectedwith the maxillae by transverse bones; and, by the pterygoids,with the mobile quadrate bones. Hence the connection of thepalato-maxillary apparatus with the other bones of the skull iaalways less close in Ophidia than in Lacertilia, and sometimesit is exceedingly lax.


5. The two rami of the mandible are united at the symphysisonly by ligamentous fibres, which are often extremely elastic.


6. The hyoidean apparatus is very rudimentary, consistingonly of a pair of cartilaginous filaments, which are united togetherin front, and lie parallel with one another beneath thetrachea. They have no connection with the skull.

These are the most apparent differences between theOphidian and the Lacertilian skull. But there are others, ofa less obvious but more remarkable character, by which theskulls of the Ophidian depart not only from that of the Lizard,but from that of other Vertebrata. Thus the basi-sphenoidpasses in front of the sella turcica, into a great rostrum,which extends forward to the ethmoidal region, and probablyresults from a parasphenoidal ossification. In many adult Ophidia two cartilaginous rods he in grooves on the upperface of this rostrum, and pass behind into the basisphenoid,while in front they are continued into the cartilaginous ethmoidalseptum. These rods are the trabeculae cranii of the foetus,which do not become united in Snakes, as they do in all theother abranchiate Vertebrata. The roof and side-walls of theOphidian skull are completed in front of the occipital segment,by two pairs of bones, which appear to be parietals and frontals.The "frontal" bones not only completely wall in thesides of the frontal region, but extend inward below, and meetin the middle line, above the basisphenoidal rostrum and thepersistent trabeculse. The "parietals" unite suturally withthe basisphenoid. These relations are not usual in true frontalsor parietals (though the latter unite with the basisphenoidin Cholonia, and the frontals unite in the middle line of thefloor of the skull in some Mammals); and as there are onlytwo bones in the place of four in this region of the skull, itbecomes a matter for inquiry whether the two bones, on eachside, respectively represent orbitosphenoids + frontals, andalisphenoids + parietals; or whether they represent overgrownfrontals and parietals only; or whether, lastly, they arethe result of an excessive development of the orbitosphenoidsand alisphenoids, true frontals and parietals being absent.According to Rathke's elaborate investigation into the developementof the skull in Coluber natrix, the two bones on eachside are formed from single centres of ossification, which appearin patches of "cartilage," which are situated, at first, inthe superolateral regions of the skull, in the place normallyoccupied by orbitosphenoids and alisphenoids, and that thesegrow up and meet in the middle line. In this case the bonesin question are orbitosphenoids and alisphenoids, and Ophidia have no true frontals or parietals; but the existence of soremarkable a deviation from the ordinary construction of thevertebrate skull cannot be admitted until the development ofthe Snake's skull has been carefully reexamined.

The Ophidia usually possess well-developed post-frontals,and they have large membrane-bones in front of the orbit,which lie upon the cartilaginous nasal chambers, and are ordinarilyregarded as lachrymals. Large nasals lie upon theupper surface of the nasal capsule between the lachrymals;and, forming the floor of the front part of the nasal chamber,on each side, is a large concavo-convex bone (Tl, Fig. 72),which extends from the ethmoidal septum to the maxilla, protectsthe nasal gland, and is commonly termed a turbinal,though, if it be a membrane-bone, it does not truly correspondwith the turbinals of the higher Vertebrata. The squamosalsare usually well developed. There is no jugal, or quadrato jugal.

Though the general conformation of the skull in the Opihidia is that which has now been described, it presents remarkablemodifications in different members of the order, especiallyin the form and disposition of the bones of the jaws. In thegreat majority of the Ophidia, the elongated palatine boneshave their long axes longitudinal, lie on the outer sides of theinternal nasal apertures, and do not enter into the formationof the posterior boundaries of those apertures. Each is connectedby a transverse bone with the maxilla, which lies at theside of the oral cavity; and the pterygoids diverge posteriorlytoward the quadrate bones, with which they are connected byligaments.

But, in the remarkable group of the Typhlopidae, the slenderpalatine bones meet upon the base of the skull in the middleline, and are directed transversely, in such a manner asto bound the posterior nasal apertures behind, as in the Satrachia.There is no transverse bone. The pterygoids lie parallelwith one another under the base of the skull, and are notconnected with the quadrate bones. The maxillae are shortplates of bone which are connected with the outer extremitiesof the palatine bones, and are directed obliquely toward themiddle line of the oral cavity, into which their free edges,armed with teeth, depend.

Again, the first-mentioned, or typical, form of Ophidianskull exhibits two extreme modifications, between which lieall intermediate gradations. At the one end of the scale arethe non-venomous Snakes, and especially Python and Tortrix (which belong to the division Aglyphodontia); at the otherthe poisonous Snakes, and especially Crotalus (Solenoglyphia).

Thus, Python (Figs. 73 and 73) has well-marked premaxillae,large maxillary bones, palatine boneswhich are firmly united with the pterygoids,and transverse bones which bindthe maxillaries and palato-pterygoidbars into one solid framework.

The maxillaries give attachment toa long series of recurved teeth, which are not very unequal in size. And Python (like Tortrix, but unlike all other Ophidia) possesses teeth in the premaxillae.

The squamosal bones are very long,and adhere to the skull, upon whichthey are slightly movable, only by theiranterior ends; and the quadrate bonesare borne upon the posterior ends ofthe squamosals, and are thus, as it were,thrust away from the walls of the skull, The rami of the mandible are looselyconnected by an elastic symphysial ligament.Thus, not only can these ramibe widely separated from one another,but the squamosal and quadrate bonesconstitute a kind of jointed lever, thestraightening of which permits of theseparation of the mandibles from thebase of the skull. And all these arrangements, taken together,allow of that immense distention of the throat which is requisitefor the passage of the large and undivided prey of theserpent.

In Tortrix, this mechanism does not exist, the short quadratebone being directly articulated with the skull, while thesquamosal, like the post-frontal, is rudimentary. The maxillarybones are also almost fixed to the skull.

In the Rattlesnakes (Crotalus, Fig. 74), the premaxillaeare very small and toothless. The maxillary bone has nolonger the form of an elongated bar, but is short, subcylindrical,and hollow; its cavity lodges the fossa formed by the integumentin front of the eye, which is so conspicuous in these,and sundry other, poisonous Snakes. The upper and innerpart of the maxilla articulates with a pulley-like surface furnishedto it by the lachrymal, so that the maxilla plays freelybackward and forward upon that bone. The lachrymal, again,has a certain amount of motion upon the frontal. The upperedge of the posterior wall of the maxilla is articulated by ahinge-like joint with the anterior end of the transverse bone,which has the form of an extremely elongated and flattenedbar connected posteriorly with the pterygoid.

The latter is long and stout, and, as usual, is united, benind,with the distal end of the quadrate bone. In front of,and internal to, its union with the transverse it is prolongedforward, and becomes united, by a movable joint, with theshort palatine bone, which is flattened from side to side,and lies on the outer side of the posterior nasal aperture. Itsanterior end is connected only by fibrous tissue with the baseof the skull. The inferior edge of the palatine bears a fewsmall teeth, and other sharp, recurved, solid teeth are attachedto the under-surface of the anterior moiety of the pterygoid.

When the mouth is shut, the axis of the quadrate boneis inclined downward and backward. The pterygoid, thrownas far back as it can go, straightens the pterygo-palatine joint,and causes the axes of the palatine and pterygoid bones tocoincide. The transverse, also carried back by the pterygoid,similarly pulls the posterior part of the maxilla, and causesits proper palatine face, to which the great channelled poisonfangsare attached, to look backward. Hence these fangs liealong the roof of the mouth, concealed between folds of themucous membrane. But, when the animal opens its mouthfor the purpose of striking its prey, the digastric muscle, pullingup the angle of the mandible, at the same time thruststhe distal end of the quadrate bone forward. This necessitatesthe pushing forward of the pterygoid, the result of which istwofold; firstly, the bending of the pterygo-palatine joint;secondly, the partial rotation of the maxillary upon its lachrymaljoint, the hinder edge of the maxillary being thrust downwardand forward. In virtue of this rotation of the maxillary,through about a quarter of a circle, the dentigerous Tace of themaxilla looks downward, and even a little forward, instead ofbackward, and the fangs are erected into a vertical position.The snake "strikes:" by the simultaneous contraction of thecrotaphite muscle, part of which extends over the poisongland,the poison is injected into the wound through the canalof the fang; and, this being withdrawn, the mouth is shut, allthe previous movements are reversed, and the parts return totheir first position.

No Ophidian possesses any trace of anterior extremities,but the Typhlopidae, the Pythons, Boas, and Tortrices, haverudiments of a pelvis, and the latter Snakes even possess veryshort representatives of hind-limbs terminated by claws.

The teeth of the Ophidia are short and conical, and becomeanchylosed to the bones by which they are supported.They may be developed in the premaxillaries, maxillaries,palatines, pterygoids, and the dentary piece of the mandible,but their presence in the premaxillaries is exceptional. In Uropeltis and some other genera, there are no palatine teeth;and in the egg-eating African snake, Rachiodon, the teethare small and rudimentary upon all the bones which usuallybear them. But the inferior spines of eight or nine of theanterior vertebrae are long, and tipped, at their apices, with adense enamel-like substance. These project through thedorsal wall of the oesophagus into its cavity, and the eggs,which are swallowed whole, are thus broken in a position inwhich all their contents must necessarily be saved.

In the majority of the non-venomous Snakes the teeth aresimply conical, but in the others, and in all the poisonousSnakes, some of the maxillary teeth (which are usually longerthan the rest) become grooved in front. In the Solenoglyphia,or Vipers and Rattlesnakes, the maxillary teeth are reducedto two or three long fangs, the groove in the front of whichis converted into a canal open at each end, by the meeting ofits edges. The teeth of the Snakes are replaced by otherswhich are developed close to the bases of the old ones.

Ophidia are not known in the fossil state before the oldertertiaries.