1864.) DR. J. E, GRAY ON THE TRIONYCHIDE. 79 
by him. They may be retained for some of the species which he 
referred to his badly characterized groups. 
Professor Agassiz, in describing the genera of the North American 
Terrapins, places great confidence in the form of the alveolar surface 
of the jaws, and probably correctly, as he seems to have studied this 
part in a series of specimens of different ages. My observations 
would lead me to believe that it is of considerable importance in the 
distinction of the Trionychide ; but there are two specimens which 
I have received from the same locality (which are in other respects 
so much alike that I am inclined to believe that they are skulls of 
the half-grown and adult animals of the same species) that are so 
different in the form of the alveolar surface as to induce me to 
believe that this part alters considerably during the growth of the 
animal, at least in some species of the family; nevertheless addi- 
tional specimens may show that what I have taken for alterations 
in growth are, in fact, specific distinctions. The examination of the 
skulls of the half-grown and the adult Tyrse nilotica and Trionyx 
gangeticus, the only species that I have at present the power of exa- 
mining in more than one state of growth, does not reveal any great 
change in the form of the alveolar surface as the animal increases 
in age. But there is no reason why a change of this kind may not 
take place in one species or genus, and not occur in others or in the 
generality of the species. I have therefore for the present adopted 
Professor Agassiz’s views. 
He seems to use the form of the alveolar edge as of generic im- 
portance, and this when he says he has a series of skeletons from 
animals of different ages. He describes as follows :— 
“Thus, the alveolar edges of the lower jaw of Amyda and Aspi- 
donectes ave sharp all round.’’— Conérib. pp. 398 & 403. 
In Platypeltis “the lower jaw, like the upper, has a very broad 
alveolar surface ; this surface is nearly flat at the symphysis, but has 
a deep depression near the hinder end.’”’—Contridb. p. 400. 
Yet these are just the characters that one might expect to occur 
after examining the skull of Cyclanosteus senegalensis, between the 
young and adult specimens of the same species. 
The examination of the series of specimens at my command in- 
duces me to place considerable confidence in the characters furnished 
by the general form of the skull—in the position of the internal 
nostrils, whether they are placed in a deep or a shallow groove in 
the palate, and if that groove is situated only behind the internal 
nostrils, or is continued in front to the edge of the jaws, and, if so 
continued, whether it is nearly of the same width throughout its 
length, or more or less contracted in front of the interior nostrils— 
and also in regard to the position of the internal nostrils themselves, 
whether they are in the front of the palate or some distance from 
the front edge, so as to be nearly on a level with the front edge of 
the zygomatic arches. 
The genera, for example, may be divided into two groups by the 
forms of the skulls, which probably indicate some peculiarity in their 
