128 DR. J. E. GRAY ON THE CHELYDIDZ. [Mar. 8, 
and a lateral series of tubercles, forming three short crests; the 
thorax oblong, covered with three short, continuous keels; marginal 
shields rather narrow, elongate ; sternum cross-like, small compared 
with the dorsal disk, narrow, slightly rounded before, acute behind, 
united to the dorsal disk by a narrow lateral process ; sternal plates 
seven, thin, four pairs and a single odd one behind; the first pair 
elongate, longer than broad (probably the two first pair of other 
Emyde united) ; the second pair broad, produced on the side, so as 
to cover the greater part of the cross-like sternum; the third pair 
elongate, narrow; the hinder plate rhombic, rather longer than 
broad, acute in front and behind; the axillary and inguinal plate 
large, covering the space between the outer lateral edge of the second 
pair of shields and the marginal plates. The front lobe of the 
sternum is very moveable at the suture between the first and second 
pairs of sternal plates, in the young specimen, and has a consider- 
able amount of mobility in the adult specimen. 
The shell is brown; the head is dark olive; the temple and the 
side of the neck pale-marbled; underside of the limbs whitish. 
Wagler represents the anal shields of S. triporcatus as divided. 
In the large specimen in the British Museum they are united into a 
single rhombic shield, as in S. salvinii. 
10. On tHE GENERA OF CHELYDID£ AND THE CHARACTERS 
FURNISHED BY THE STUDY OF THEIR SKULLS. By Dr. 
J. E. Gray, F.R.S., erc. 
It has been very generally observed that the characters which 
separate the genera of the family Chelydide are very slight. This 
only arises from the genera having been hitherto characterized by 
some easily seen external peculiarities, which are often, as in this 
case, mere superficial indications of very different internal organiza- 
tions. 
This apparent slightness disappears when the skulls and other 
parts of the skeletons of the different genera are examined, as may be 
proved by consulting the figures of the skulls and skeletons given in 
the Atlas of Plates to Wagler’s ‘System of Amphibia,’ published in 
1830, which has been too much neglected by more recent writers on 
the subject. 
Unfortunately we have the bones cf only a few examples of the 
family in the collection of the British Museum, and there are only 
two skeletons in the Museum of the College of Surgeons ; nor do 
I know of any other osteological collections which have more. 
These, however, and the figures of Cuvier and Wagler before referred 
to, are sufficient to show the outlines of an improved arrangement of 
the genera, and to afford more important characters for them. 
I refer to my ‘Catalogue of the Shield Reptiles in the British 
Museum’ for the description of the species and more lengthened 
generic characters, and to the articles by me in the ‘ Proceedings of 
