590 CHORDATA. 



maxillaries and maxillaries. Extremely characteristic of the 

 reptiles, the turtles excepted, is an os transversum, which appears 

 in no other vertebrates. It extends from the hinder end of the 

 maxillary to the pterygoid (figs. 619, 626, 627, 630, Ts, tr). A 

 jugal is also frequently present. Of the other visceral 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, the four occipital bones being present. The basi- 

 occipital forms the larger part of the single occipital condyle, in 

 which parts of the exoccipitals participate, the single condyle 

 being the sharpest distinction between the reptilian and amphibian 

 skull. The basisphenoid, which lies in front of the basioccipital, 

 has an anterior process or rostrum, representing the rudimentary 

 parasphenoid (possibly presphenoid). Above, the skull is roofed 

 in with membrane bones : parietals (frequently fused and perforated 

 by the parietal foramen for the pineal eye), frontals, nasals, as 

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

 mals as well. 



The ethmoidal region is largely cartilaginous ; ali- and orbitosphenoids 

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

 epiotic 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 quad rat ojugal, while above it may be a 

 second arch formed of postorbital and squamosal. 



The convex occipital condyle forms, with the concave surface 

 of the first vertebra (atlas), an articulation for motion in the ver- 

 tical plane and lateral motions, while a twisting around the long 

 axis of the body is permitted 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 body of 

 the axis, forming a pivot around which the atlas turns. There are 

 two sacral vertebrae, and the vertebras 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. When present 

 the number of digits varies between three and five (usually four or 

 five). In the pelvis ischium and pubis are separated by an obturator 

 foramen and are united with the corresponding bones of the oppo- 

 site side by a double symphysis. In the shoulder girdle scapula 

 and coracoid alone are constant, a clavicle occurring in turtles and 

 lizards, in the latter an episternum (fig. 564) as well. Of con- 

 siderable systematic importance is the position of the ankle joint. 



