DERIVATION OK ORDER. 



2 9 



in Diadectes the temporal roof is notcht where applied to the quadrate and the latter 

 is excavated to form a tympanic cavity; instead of a pair of prevomers, there is a 

 single vomer; and there is a sort of carapace composed of bony plates overlying 

 several pairs ot the ribs. 



It seems evident that we are getting close to the base of the phylogenetic tree of 

 the chelonians; but, likewise, that we have not yet reacht it. Taking into con- 

 sideration the fact that no turtle possesses a temporal fenestra, that when the 

 temporal roof is deficient either the zygomatic arch or the parietal arch or both are 

 missing, it seems impossible to derive the turtles from any group of reptiles in which 

 even the beginning of such fenestra? has been made. If this view is correct, there are 

 excluded at once from the chelonian ancestry all those reptiles belonging to Professor 

 Osborn's Diapsida and all the Anomodontia, limited so as not to include the 

 Cotylosauria. The Diadectida?, too, would be excluded, if they really possest 

 temporal fenestra?. If the presence of a pair of prevomers, instead of a vomer, has 

 the importance attributed to it by some recent authors, the Cotylosauria, as limited 

 by Case, would have to forego their claims. Of known reptiles we appear to be 

 limited to the Diadectida? and the Otoccelida?, in our search for the ancestors 

 of the chelonians. 



It is improbable that any of the reptiles of the Permian of Texas or equivalent 

 deposits were the ancestors of the turtles. When we consider that already in the 

 Upper Trias the turtle Proganochelys possest a typical shell, we must conclude that 

 the earliest of the race must have been in existence as early as the Permian itself. 

 It is, of course, possible, indeed quite probable, that some earlier, less differentiated 

 form of one or the other of the two families just mentioned gave origin to the most 

 primitive turtles. 



It is a serious objection to any of the Cotylosauria, using the name in the wider 

 sense, that have been mentioned as possible ancestors of the turtles, that they appear 

 to have possest no ventral armor. Almost certainly the turtles inherited this portion 

 of the shell, in some form, from the Stegocephalia. The Diadectes described by 

 Case presented no traces of a plastron, or of abdominal ribs. There are various 

 other reasons why this reptile can not pose as the founder of the chelonian line. 

 Among these are the elevated spines of the vertebra?, the great amount of motion 

 between the several dorsal vertebra?, their complex structure, the anchylosis of the 

 sacrals, and the fact that the plates of the carapace lie between the scapula and the 

 ribs. In turtles the scapula lies inside, not outside, of the costal plates. 



Dr. O. Jaekel, of Berlin, has described (Neues Jahrb. Min., 1, 1902, p. 127) an 

 interesting genus named by him Placochelys and regarded by him as standing in 

 ancestral relations to the turtles. The body of the animal is short and broad, and is 

 covered by an armor of thick small plates, all closely joined. There is no ventral 

 armor. The specimen was found in the lower portion of the Keuper. The well- 

 preserved skull shows that there were large supratemporal fenestra? and that the 

 nasal openings were separate and far behind the end of the snout. These structures, 

 especially the first named, make it improbable that this reptile is at all closely 

 related to the turtles. 



The discovery of Otocalus and Diadectes and Placochelys is like the capture of 

 so many stragglers and deserters from an army of heterogeneous composition. 

 From these prisoners we may learn something of the personnel and the equipment 

 of that army; but it would be unsafe to draw from the data obtained too wide 

 conclusions. From Placochelys we learn that in Triassic times there were broad and 

 short bodied reptiles of tortoise-like form and covered with an armor of small plates. 

 From Diadectes and Otoccelus we discover that in Permian times there were more 



