1903.] HAY—EXISTING GENERA OF THE TRIONYCHIDA. 273 
already said, Baur recognized it as the type of Dogania ; but if it is 
a member of a genus distinct from Aspidonectes, it must be called 
Amyda, according to the systems of both Bonaparte and Fitzinger. 
Platypeltis comes next, having as its type TZestudo ferox 
Schneider. It will include all the American soft-shelled tortoises, 
except Aspidonectes californiensis (Rivers). The writer believes 
that this group is sufficiently characterized by the possession of only 
seven pairs of costal plates. The smooth or granular condition of 
the skin of the young is possibly a character of generic value. In 
this group must be included LeSueur’s Zyrionyx muticus. There 
appear to be no characters which justify its separation as a distinct 
genus. Baur makes it the type of Amyda, following Agassiz. The 
only character given by Baur to distinguish it from P/atypeltis 
spiniferus, for instance, is the separation of all the costals at the 
midline by means of neurals; whereas in the other American 
Trionychidz the hindermost pair are in contact. This difference 
depends wholly on the greater or less development of the seventh 
neural plate; and this will almost certainly be found to vary in 
different species and in different individuals of the same species. 
Some importance has been attributed to the absence in mudicus of 
the commonly occurring ridges, or papilla, on the septum of the 
nares; but this character appears te the present writer to be of 
slight value. On similar characters the Trionychidz might prob- 
ably be divided into as many genera as there are species. If, how- 
ever, Zrionyx muticus is to form a distinct genus, a new generic 
name must be coined for it. 
For Boulenger’s group I, B, 1, Dr. Baur accepted Gray’s generic 
name /so/a, having, according to Baur’s statement, 7rionyx letthit 
as its type. This is, however, an obvious error. The genus was 
proposed by Gray in 1873 (Proc. Zool. Soc. Lond., p. 51) for the 
reception of Z7rionyx peguensis Gray, and this is, according to 
Boulenger, a synonym of Zxeonyx formosus. T. leithit was after- 
_ward (Ann. Mag. Nat. Hist. [4], x, p. 157, 1873) referred to the 
same genus with some doubt. J/so/a is therefore a synonym of 
A spidonectes, as recognized in the present paper. 
The group of tortoises referred by Baur -to /so/a includes the 
species gangeticus, hurum and /eithit. These species differ from 
those of Aspidonectes, especially in possessing two neural plates 
between the first costals. It appears to be worthy of generic rank. 
A search among the generic names which have been applied to the 
