MODIFICATION IN TURTLES. 19 



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 Pseudotrionyx, 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. 1 his 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 Cheloniidae 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 Emydidae 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, in others nothing is left but the postorbital bar. Several genera 

 possess a parieto-squamosal arch (Rhinemys, Hydrastis, 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 removal of the 

 bone has beeun 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 choanae opened 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 choanae be pusht backward. This was accomplisht by the development of 

 a median descending plate of the vomer, w T hich spread laterally and joined palatal 

 plates from the maxillae, 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 Crocodiha; only, in the most primitive known forms 

 of the latter the choana? are placed immediately in front of the pterygoids, while in 

 the most advanct forms the choana? 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 of greater extent among the 

 turtles characterize families only or even genera. 



