AND IRREVERSIBLE EVOLUTION. 1051 



well ossified with spherical suifoces of articulation {Divietrodon). 

 The latter tj^pe is to be found also in all terrestrial Squamata 

 that have well-developed feet. Very little is known until now 

 about the foot-bones of the Theromorpha ; they seem, however, 

 always to have attn.ined a high degree of perfection. 



Turning from the monozygocrota,phous Theromorpha to the 

 likewise nionozygocrotnphous Sauropteiygians, which evidently 

 descended from Pelycosaurians or Theromorpha, one is surprised 

 to remark that even in those Sauropterygians that are least 

 adapted to aquatic life {Neusiicosaurus, Proncusticosaurus (text- 

 fig. 8 (14)) the foot-bones are flat and i-ounded ossicles that evi- 

 dently formed the centres of cartilaginous, polygonal plates. The 

 same sort of foot-bones are present in the liassic Plesiosaurians. 

 In the more specialised later Plesiosaurians, instead of the cartila- 

 ginous plates, polygonal flat bones are present. The flat polygonal 

 bones observable in Elasmosaurus, Polycotylus (text-fig. 8 (15)) 

 or Gimoliosaurns recall somewhat the flat polygonal bones of the 

 specialised Stegocephalians. 



Siniilivr changes as those in the Sauropterygians can also be 

 observed in the Ichthyopterygians. In Mesosaurits and some 

 triassic Ichthj^osaurians {Shastascairus, Delphinosaurus (text-fig. 8 

 (12)) round bony disks occur that were evidently surrounded 

 by extensive cartilage. In all the more specialised Ichthyo- 

 saurians (text-fig. 8 (13)) polj^gonal bony plates are present. 

 These are firmly applied against each other. 



Somewhat similar changes as in these groups are to be met 

 with in the Testudinata. In the terrestrial tortoises polygonal 

 bones occur with a small amotmt of mobility between them. In 

 the TrionychifUo these bones shoAv by retaining at their angles 

 a good amount of cartilage a tendency to round oflf these angles. 

 In the Chelonidse (text-fig. 8 (17), Toxochehjs) this process is still 

 more marked ; finally, in Dermochelys (text-fig. 8 (16)) instead of 

 angular bones, polygonal plates of cartilngo are present, with 

 flat bony disks in the centre. These changes show that the 

 rounding oflf of the primitive polygonal foot-bones of the Testu- 

 dinata is due to the retention of an embryonic stage throughout 

 life (14). 



Applying this observation to the fossil marine forms hitherto 

 discussed, it becomes obvious that also in these first a cartila- 

 ginous embryonic stage, with small centres of ossification, had 

 become permanent for some time, and that after this transitory 

 stage extensive ossification set in, in much the same manner 

 as in the most primitive reptiles. In this instance it will be 

 remembeied that the hypothesis of a secondary ossification has 

 already been brought forward in the foregoing paragraph. 



Among the marine Squamata the carpal and tarsal bones 

 retain spherical articulating surfaces in Opetiosixurits, they have 

 become to a great extent cartilaginous in Tylosaurus (text-fig, 8 

 (18)), and are already to some extent replaced l^y flat polygonal 

 bones in Plaiecarjms (text-fig. 8 (19)). So also in this gi^oup the 

 same changes occur as in the groups already dealt with. 



