332 THE STRUCTURAL EVIDENCE OF EVOLUTION". 



Anura 



Urodela Trachjstomata * 



I 

 Proteida 



Stegocephali 



I. 

 Embolomeri Rhachitomif 



Ganocephala J 



An examination of the above tables shows that there has been 

 in the history of the Batrachian class a reduction in the number 

 of the elements composing the skull, both by loss and by fusion 

 with each other. It also shows that the vertebrae have passed from 

 a notochordal state with segmented centra, to biconcave centra, 

 and finally to ball-and-socket centra, with a great reduction of the 

 caudal* series. It is also the fact that the earlier forms (those of 

 the Permian epoch) show the most Mammalian characters of the 

 tarsus and of the pelvis. The latter forms, the salamanders, 

 show a more generalized form of carpus and tarsus and of pelvis 

 also. In the latest forms, the Anura, the carpus and tarsus are 

 reduced through loss of parts, except that the astragalus and cal- 

 caneum are phenomenally elongate. We have then, in the Batra- 

 chian series, a somewhat mixed kind of change ; but it princijDally 

 consists of concentration and consolidation of parts. The ques- 

 tion as to whether this process is one of progression or retrogres- 

 sion may be answered as follows : If degeneracy consists in **the 

 loss of parts without complementary addition of other parts," then 

 the Batrachian line is a degenerate line. This is only partly true 

 of the vertebral column, which presents the most primitive char- 

 acters in the early, Permian, genera (Rhachitomi). If departure 

 from the nearest approximation to the Mammalia is degeneracy, 

 then the changes in this class come under that head. The carpus, 

 tarsus, and scapular and pelvic arches of the Rachitomi are more 

 Mammalian than are those of any of their successors. || 



* The Trachystomata probably came from the Urodela by a process of degener- 

 acy. See "American Naturalist," Dec, 1 885. (Ed. 1886.) 



f Includes the Eryopidae. 



X Includes Trimcrorhachidae and Archegosauridae ; and is distinguished from the 

 Rhachitomi only by the presence of a single and cotyloid articulation of the skull 

 with the atlas. 



* This reduction extends to the dorsal series as well. (Ed. 1S8G.) 



11 This should read, than their latest, or anurous successors. (Ed. 1886.) 



