SKELETON. 



8 9 



species. The branchial arches are usually better developed than the hyoid proper, 

 which is cartilaginous in most snakes and is lacking in the crocodiles. In the 

 chelonia (fig. 93) two branchial arches are usually present. 



The THERIOMORPHS (fig. 90) have a short, broad skull with parietal foramen; 

 and that of the cotylosaurs was much like that of the stegocephals. In the more 

 differentiated groups the skull recalls that of mammals, especially in the partici- 

 pation of the squamosal in the hinge of the jaw. Lacrimals are occasionally 

 absent, sclerotics sometimes present. The palatal region is known in a few forms. 

 The pterygoids may meet only in front, leaving a vacuity between it and the 

 basisphenoid, or they may meet that bone. The choanae are in front of the pal- 

 atines but (theriodonts) may be displaced backward by palatine processes of the 

 maxillaries. 



All four occipitalia are developed; the occipital condyle is tripartite, being formed 

 by basi- and exoccipitals, but in Cynognathus the recession of the basioccipital 

 results in a dicondylic condition. The greatest variations occur in the temporal 



FIG. 90. Skull of Procolophon, after Woodward. For letters see fig. 68. 



region. In the lower cotylosaurs the cranial roof is without fossae (Broom doubts 

 the infratemporal fossa of Procolophon}. In other theromorphs quadratojugal 

 and supratemporal are lacking, the squamosal meeting the parietal. Placodus 

 has only the supratemporal fossa, but in the majority the upper arcade has dis- 

 appeared, leaving a large temporal vacuity, much as in mammals. 



Little is known of the lower jaw. The bones are sometimes discrete, sometimes 

 extensively fused. The teeth are thecodont, and in the theriodonts are differentiated 

 into incisors, canines and molars, but in the anomodonts teeth are absent, or at most 

 there are a pair of large incisors in the upper jaw. 



In the PLESIOSAURS and their allies the skull is about a twelfth of the total length. 

 There is a parietal foramen between the parietals, which have a process for artic- 

 ulation with the squamosal, the supratemporal being absent. The large pre- 

 f rentals intervene between the f rentals and the orbits; lacrimals and usually 

 nasals are absent. The large temporal fossa in bounded externally by the jugal 

 which extends back to the quadrate. The choans are in front of the palatines; an 

 os transversum is present and there is frequently a parasphenoid in the inter- 

 pterygoid vacuity. All have a subtemporal vacuity and there is another in the 



