26 FOSSIL TURTLES OF NORTH AMERICA. 



sible. None of the superfamily has nasals, lacrimals, or temporal roofing. Their 

 derivatives must have acquired these bones. The neck is most completely of all 

 turtles adapted for retracting the head within the shell. To give origin to the neck 

 of the Pleurodires this neck must have lost its peculiar mechanisms and acquired 

 those of the side-neckt or snake-neckt turtles. To have produced the neck found 

 in the Amphichelydia, that of the Trionychoidea must have shortened greatly 

 and have changed its biconvex and its concavo-convex vertebrae into biconcave 

 ones. The Amphichelydia, the Cryptodira, and the Pleurodira must have developt 

 peripheral bones, instead of inheriting them from their ancestors. The Amphiche- 

 lydia and many Pleurodires did not inherit their mesoplastral bones, but acquired 

 them independently. The limbs of those Thecophora that are fitted for walking 

 must, according to the scheme of derivation proposed by Haeckel, have been evolved 

 from feet fitted for swimming. Turtles endowed with a covering of horny scutes 

 came from a race which are wholly devoid of these coverings. All these procedures 

 are the exact reverse of what is generally believed to be the course followed by animals 

 in their evolution. If it be claimed that the Trionychoidea of that early time possest 

 nasals, lachrymals, and a temporal roof, that the neck was yet short and composed 

 of biccelous vertebras, that they had peripheral bones and mesoplastra, then they 

 were not Trionychoidea at all, but Amphichelydia or something very close to them. 



It is a difficult matter to estimate properly the relative rank of the three super- 

 families of the Thecophora the Pleurodira, the Cryptodira, and the Trionychoidea. 

 As to the Amphichelydia, there can be no doubt that this group ranks below all 

 the others and that from it have been derived all the others. It appears that the 

 Pleurodira are usually regarded as the most specialized turtles. There is no doubt 

 that the skull and the pelvis have departed farther from those of the Amphichelydia 

 than have those of either the Cryptodira or the Trionychoidea. The most im- 

 portant modification in the skull is the posterior shortening of the pterygoids, 

 whereby the basisphenoid is permitted to join the quadrate. Likewise the outer 

 anterior border of the pterygoids has become rolled up in a peculiar manner. On 

 the other hand, the skull of a number of genera has retained the nasals and the 

 posterior notch in the quadrate, both primitive features. It seems to the writer that 

 the neck is less specialized than that of the Cryptodira and Trionychoidea. The 

 shell has undergone far less specialization than that of the other groups mentioned, 

 many of the genera retaining the mesoplastra, elements unknown in the others. 

 On the whole, the writer is inclined to place the Pleurodira below both the Crypto- 

 dira and the Trionychoidea. 



As regards the Trionychoidea, it is believed that the skull has departed further 

 from the amphichelydian pattern than has that of most of the Cryptodira. This is 

 seen in the universal reduction of the temporal roof to narrow postorbital and 

 zygomatic arches, the backward prolongation of the squamosal processes, and the 

 closure of the stapedial notch in the quadrate. Altho the articular ends of the cer- 

 vicals present, so far as is known, less variety of form than in the Cryptodira, the 

 neck is, as has been said by Boulenger, more perfectly adapted for complete and 

 rapid retraction than in any other chelonian. The carapace has become greatly 

 specialized through degeneration of the peripherals and of the horny scutes. The 

 limbs have become moderately specialized for swimming. The Trionychoidea can 

 hardly rank below the Cryptodira; it is convenient to let them, in a scheme of 

 classification, follow the group just mentioned. 



In fig. 8 an attempt has been made to indicate the connections between the 

 different families of turtles. This chart differs in some respects from the one pub- 

 lisht by the present writer in the Bulletin of the American Museum of Natural 



