466 SCIENCE PROGRESS 



" turtle " being apparently applied in the States to any member of 

 the chelonian order, and not restricted, as in this country, mainly 

 or exclusively to the marine species. The study of American fossil 

 tortoises (to use the term more generally in use) dates only 

 from 1 85 1, when the late Prof. Leidy named and described four 

 species. Dr. Hay's memoir contains an account of no less than 

 266 species, of which 76 are described for the first time. The 

 author has some interesting remarks with regard to the origin of 

 the chelonians : that they are not derived from the Cotylosauria 

 or Pelycosauria seems evident, owing to the apparent absence in 

 that group of a ventral armour, which was certainly wanting in 

 Diadectes. The genus Placochelys of the European Trias has 

 been regarded as ancestral to the Chelonia ; but, although the 

 body is short and broad and curved above with an armour of 

 small, closely approximated plates, there is no ventral shield. 

 And it accordingly seems that we must look to the labyrin- 

 thodonts (stegocephalians) as the ancestral stock. 



" From Placochelys" writes Dr. Hay, " we learn that in Triassic 

 times there were broad and sharp-bodied reptiles of tortoise-like 

 form and covered with an armour of small bony plates. From 

 Diadectes and Otoccelus we discover that in Permian times 

 there were more elongated reptiles, in the skulls of which 

 there were many chelonian characteristics, and whose bodies 

 were protected by an armour of elongated plates overlying 

 the ribs. The stegocephalian genus Archegosaurus, of the 

 Permian, possessed a ventral armour of elongated plates. We 

 may confidently expect that in the Permian there will yet 

 be found an animal possessing such a combination of these 

 characters, together with other features, that we can recognise 

 in it the ancestor of the order of turtles." 



Neither does the author attach much importance to the 

 theory that the chelonians are closely allied to the plesiosaurs. 

 The most that, in his opinion, can be affirmed is that possibly 

 the ancestral stocks of the two orders were somewhat nearer 

 to one another than those which gave rise to the other orders 

 of reptiles. 



In connection with the supposed affinity of the Chelonia 

 to the Sauropterygia (plesiosaurians) reference may be made 

 to a paper by Mr. R. L. Moodie on reptilian epiphyses, pub- 

 lished in the American Journal of Anatomy, vol. vii. p. 443. 

 It has been stated that the limb-bones of both chelonians 



