AND IRUEVERSIULK KVOLUTION. 1049 



Siij)})0.siiig that in J)ermoclid>/s the whole of tlie c;iriilnge 

 were to turn to bone, very soon a, pelvis would evolve thnt would 

 recall the most primitive reptilian pelvis. It would be more 

 primitive than the pelvis of the Ampliichelyda^ That sucli a 

 reversal can actually take place will be proved in detail in the 

 following parngraph, here it is enough to emphnsise that in 

 rteranodon such an ossification acttially did occur. 



On account of the complexity of the changes in the ventral 

 elements of the pelvis of i-eptiles the diagram also becomes 

 complex. 



Ventral pnrt of pelvis to Hod-like bones 



(jreat extent cai-tilaffinovs, Honj/ plates with small with large 



foramen ohturatoritini small. foramen obturatorinm. apertui'es. 



Primitive Stegoecpl.alia >- [ Spccialisptl Stesoceplialia; 



(. iiiiniitivc reptiles 



Primitive marine (and 7. < 

 volant) reptiles 3 \ 



-->• <i SjiGcialised 

 (. icptiles. 



\ C Spodialiscd ninrino and 

 ^ (_ volant reptiles 



§(4) Carpus and tarsus of re2ytiles. 



The changes that can be observed in the carpus and tarsus of 

 reptiles are similar to those in the pelvis. 



In primitive Stegocephalia, as in modern Urodeles, the carpus 

 and tarsus consist of (lat polygonal pieces of cartilage with small 

 disk-shaped ossicles in their centre. In this respect it is sufii- 

 cient to refer to Uranocentrodon and to Hclncosatirtis, the foot- 

 bones of which have been figured by Broom (3). In other 

 moi'e reptile-like Stegocephalians carpus and tarsus consist, no 

 longer of cartilage but of more or less flat polygonal bones with 

 but little cartilage between them (text-fig. 8 (10)) {Trematops). 

 '.I.'hese tarsal and carpal bones evidently originated by the ossifica- 

 tion of the whole or of nearly the whole cartilage of the primitive 

 forms Avithout much change in shajje. 



With the exception of the Procolophonida), the Cotylosauria 

 show much the same sort of foot-bones as the Stegocephalia. 

 In the primitive ones (Diadectes, Disparactus (text-fig. 8 (11), 

 Limnoscelis), evidently polygonal cartilage plates were present 

 with disk-like centres of ossification in their middle, while in the 

 more specialised ones (Pai-eiasauridfe) the cartilage is replaced 

 by polygonal bone. In the Procolophonidije the structure of the 

 foot-bones is different. Instead of cartilaginous or osseous, 

 nearly immovable elements, ossicles with well-marked concave 

 and convex surfaces of aiticulation are present. Probably a fair 

 amount of cartilage was present, Init probably also the surfaces 

 of the cartilage-bodies were curved. 



In the PeJycosaurians the carpus is still sonietimes polygonal 

 with a small amount of flexibility (Ophiacodon) , sometimes 

 rounded with a fair amount of cartilage (Varanojjs), and sometimes 



