MODIFICATION IN TURTLES. fe) 
other groups the boundaries of the scutes are not visible on the shells; but in most 
of these cases the scutes were probably very thin and their edges did not impress 
the bone. In the living Carettochelys and the extinct Penienienie yx, both Crypto- 
dires, there are no scutes. 
The principal characters which belong to turtles in general will be given on a 
succeeding page. Besides these common characters, others that have been lost or 
modified were possest by the more primitive members of the order. Of these may 
be mentioned the presence of nasals, of lachrymals, and of an extensive roof over the 
temporal region. ‘The presence of this roof in the skulls of the older turtles is so 
general, perhaps rather so universal, that its primitive nature can hardly be doubted. 
Its frequent presence in the skulls of the lower forms of living turtles and its general 
absence in the case of the higher forms confirm this conclusion. ‘This roof must have 
been inherited from the Cotylosaurian ancestors of the order. Otherwise, we must 
suppose that it was developt during the history of the chelonians and again lost by 
most of them. It appears obvious that the roof has been reduced in proportion to the 
elongation of the neck and the ability to withdraw the head beneath the shell. In 
Dermochelys and the Cheloniide the roof extends backward as far as the occipital 
condyle; and all these turtles have short necks and can furnish beneath the shell 
little protection to the head. In the snappers the roof is moderately developt; in 
most of the Emydidz and all the Trionychoidea the roof is reduced to a narrow 
postorbital bar and to a zygomatic bar. In some species of Terrapene and in some 
other genera the zygomatic bar is missing. While in some Pleurodira, as Podocnemis, 
the roof is wide, 1 in others nothing is left but the postorbital bar. Several genera 
possess a parieto-squamosal arch (Rhinemys, Hydrasprs, Hydromedusa). Dr. 
George Baur showed the various ways in which these results have been attained. 
In the Cryptodira the roof has been eaten away from behind forward and in front 
of the tympanic cavity, severing first the union of the parietal and the squamosal, 
then that of the postfrontal and the squamosal, and finally in rare cases that of the 
jugal and the quadratojugal. In the Pleurodira the reduction has followed the 
reverse course, beginning in front of the tympanic cavity and moving upward and 
backward. In several genera of Pleurodira there has been left only a very narrow 
parieto-squamosal bar; in others, not even this. In all cases the remov al of the 
bone has begun at one border or the other of the bone and never by the formation of 
fontanels in the roof. 
In no other order of reptiles do we find such enormous variation in the character 
of the temporal roof. Superfamilies and families of turtles present modifications 
that would characterize orders of other reptiles. 
The palatal region offers variations in structure that are of wide range. Primi- 
tively the choane opened i in the front of the mouth one on each side of the narrow and 
shallow vomer. A broadening of the triturating surfaces of the jaws required that 
the choanze be pusht backward. This was accomplisht by the development of 
a median descending plate of the vomer, which spread laterally and joined palatal 
plates from the maxilla, the palatines, and sometimes even from the pterygoids. 
Thus a floor was formed beneath the nasal channels, carrying these backward 
sometimes more than half-way to the occipital condyle. The process is similar to 
what occurred in the order of Crocodilia; only, in the most primitive known forms 
of the latter the choanz are placed immediately in front of the pterygoids, while in 
the most advanct forms the choanz are in the rear of these bones. These differences 
were regarded by Huxley as sufficient to justify the recognition of two groups of 
crocodiles, the Mesosuchia and the Eusuchia. Variations oi greater extent among the 
turtles characterize families only or even genera. 
