Evolution OF Animals 473 



the digits are typically 4-5; there are movable eyelids except 

 in Typhlotriton; teeth are as above; the skeleton throughout 

 is well ossified and the vertebrae are now opisthocoelous in most 

 genera, though Plethodon, Spelerpes, and other genera have 

 pseudocoelous; there is one epibranchial cartilage, and all of 

 the four larval clefts close up; the carpal and tarsal elements 

 are ossified and numerous (8-9); the alimentary canal is much 

 as in the last. 



In setting out the above characters the writer would by 

 no means assert that all of the included genera as well as their 

 groups represent a continuous and rigid line of advance. For 

 the extensive organic denudation that must have occurred 

 during past geologic periods since the permian epoch has left 

 us these only as island-groups of scattered organic continuity. 

 But the above assemblage of characters, that could be increased 

 by many additional minor ones, seem to indicate for us the 

 main pathway of urodele advance, and the sign posts, so to 

 say, that point the way. But if more detailed comparisons 

 be carried out, as, for example, along the lines of comparative 

 pelvic structure, as figured by Wiedersheim (138: 112), or of 

 pectoral structure, the correctness of the writer's views will 

 probably be granted. 



One point here deserves emphasis. If the main trend of 

 evolutionary progress toward the mammals is as has been 

 sketched, then we must look for the first beginnings of true 

 tetrapodous limbs amongst either the Apoda, or the sirenid 

 and the proteid series. The careful embryological studies 

 already made on these by Gotte, Strasser, and others should 

 be carried to all species of the component genera of the groups. 



But the objection may, at least on first glance, be perti- 

 nently urged, that the Apoda are scaled animals while the 

 main groups emphasized above have been scaleless. To this 

 it can be replied that a smaller half of the living genera of 

 Apoda are scaleless, while even the scaled genera have the 

 scales formed deeper in the body substance and in a quite 

 different manner embryologically from true scales. They 

 might well, therefore, be designated the "Pseudolepidapoda." 



