400 S. W. WILLISTON 



tive reptilian phalangeal formula was that now persistent in the lacer- 

 tilia and Sphenodon, 2, 3, 4, 5, 4. In Ceraterpeton, a later form than 

 Isodectes copei, Woodward has demonstrated the formula 2,3,4,4,3, in 

 one species at least. The number 2,3,3,3,3, so characteristic of the 

 mammals, is a late specialization, and can have, in my opinion, no 

 genetic relations with the similar formula in most turtles. In other 

 words the phalangeal formula is not of the great importance that some 

 authors have attached to it. 



The attachment of the ribs to the intercentral space is very hard to 

 explain under the supposition that the amphibian centrum is the 

 hypocentrum; if the vertebrae are really composed of the pleuro- 

 centra, the entire, or apparently entire, loss of the hypocentra in these 

 the earliest known air-breathers offers another bewildering problem. 



There are those who beheve that the reptiles arose from two distinct 

 groups of the amphibia, one from the Microsauria, the other from the 

 TemnospondyM ; and I must confess that Isodectes helps that theory 

 materially, for its relationships with the Microsauria on the one side 

 cannot be gainsaid, But, the close relationships between such forms 

 as Pariotichus, Procolophon, Telerpeton, the Pelycosauria, the Coty- 

 losauria, Pareiasauria, and Temnospondyli complicate matters here 

 exceedingly, and leave the whole subject still in great obscurity. 



