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 vertebra 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 vertebra, that they had peripheral bones and mesoplastra, then they 
were not [rionychoidea at all, but Amphichelydia or something very close to them. 
It is a dithcult matter to estimate properly the relative rank of the three super- 
families of the [hecophora—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 Cry ptodira or the Trionychoidea. The most im- 
portant modification in the skull is the posterior shortening of the pterygoids, 
Ww hereby 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 amphichely dian 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 dhelomione 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 Cry ptodira; it is convenient to let them, in a scheme of 
classification, f follow the group just mentioned. 
In fig. 8 an attempt has been made to indicate the connections between the 
different families of turtles. ‘his chart differs in some respects from the one pub- 
lisht by the present writer in the Bulletin of the American Museum of Natural 
