522 CHORDATA 



it and the cranium. Behind it is the columella, its inner end inserted in 

 the fenestra ovalis. From the quatlrate the palatine series of l)ones — 

 pter_vgoid, palatine, vomer — extends forward, these being frequently 

 toothed; and in front of and parallel to it the premaxillaries and maxil- 

 laries. Extremely characteristic of the reptiles, the turtles excepted, is an 

 OS lrj>isvcrsii)ii, wliich appears in no other vertebrates. It extends from 

 the liinder end of the maxillary to the pterygoid (ligs. 572, 578, 57Q, 581, 

 Ts, tr). A jugal is also frequently present. Of the other A'isceral 

 arches, since gills are lacking, only the hyoid bone and laryngeal carti- 

 lages persist. 



In the cranium the complete ossification of the occipital region is 

 noticeable, tlie four occipital bones being present. The basioccipital forms 

 the larger part of Lite single occipital condyle, \\\ which parts of the ex- 

 occipitals participate, the single condyle being the sharpest distinction 

 between the reptilian and ampliibian skull. The basisphenoid, which 

 lies in front of the basioccipital, has an anterior process or rostrum, rep- 

 resenting the rudimentary parasphenoid (possi]>lv presphenoid). Above, 

 the skull is roofed in with membrane bones: parietals (frequently fused 

 and perforated by the parietal foramen for the parietal eve), frontals, 

 nasals, as well as pre- and postfrontals and postorbitals, and usually 

 lacrimals as well. 



The ethmoidal region is largely cartilaginous; all- and orbitosphenoiiis are 

 small and variable. Only the prootic is constant in the otic region; ejiiotic 

 and opisthotic usually fusing with the occipitals, the opisthotic being large and 

 distinct only in the turtles. The zygomatic arch (lost in snakes) is formed of 

 jugal and quadratojugal, while above it may be a second arch formed of post- 

 orl.)ital and squamosal. The zygomatic arch bounds a gap in the skull, the 

 orbilolcniporal fossa; the other arch a snpratcmporal fossa. The tirst fossa 

 may be subdivided by a process from the zyi,'omatic to the postorbital or post- 

 frontal bones, thus giving separate orbital and temporal foss;e. 



The convex occipital condyle forms, mth the concave surface of the 

 first vertebra {alias), an articulation for motion in the vertical plane aitd 

 lateral motions, while a twisting around the long axis of the body is per- 

 mitted by the joint between the atlas and the second vertebra, the axis 

 or epistropheus. The atlas is a bony ring, its centrum having separated 

 and united with the Ijody of the axis, forming a pivot arounii which the 

 atlas turns. There are two sacral vertcl)ra', and the vertebra' of the trunk 

 are divided into thoracic and lumbar, the former bearing long ribs which 

 reach to the sternum, while the shorter ribs of the neck end freely. 



Limbs are lacking in snakes and some lizards, ^^'hen present the 

 number of digits varies between three and live. In the pelvis ischium and 

 pubis are separated by an obturator foramen and are united with the cor- 



