1864. ] DR. J. E. GRAY ON THE TRIONYCHID. ii 
peltis ferox, Aspidonectes spinifer, and A. asper is so great that I 
am not surprised that they have been confused or even deliberately 
considered as identical. We have, in fact, a case here, of which a 
few other examples only are thus far known, in which, under the 
most surprising similarity of external appearance, marked structural 
peculiarities amounting to generic differences are hidden. I have 
already pointed out such cases in the genera Phoxinus and Chroso- 
mus, and in the genera Carpiodes, Bubalichthys, and Ichthyobus 
among Cyprinoids.” (Amer. Journ. of Science, 2nd ser. xix. p. 71. 
**Many similar examples might be quoted among the Rodentia.” 
(Contrib. p. 410, note.) 
I believe that such cases are much more common than has hitherto 
been suspected ; and it is on such superficial resemblances that Mr. 
Bates’s observations and theories respecting the Brazilian Butterflies 
are founded—notions which will vanish into the air when the insects 
are more carefully examined by a systematic entomologist. 
Professor Agassiz, in the American species, points out a difference 
in the form and structure of the nostrils :— 
Thus, in Amyda mutica the nostrils are small, simple, circular, and 
far apart, rather on the underside of the snout. 
In Platypeltis ferow and Aspidonectes spinifer these are larger, 
close together, and with a process on the middle inner side of each. 
Unfortunately it is not possible to make similar observations on 
the African or Asiatic species, as one has not the power of observing 
them alive. Indeed they rarely arrive in a sufficiently good state to 
make the comparison with certainty in the specimens preserved in 
spirits. All the African and Asiatic species that I have been able to 
examine seem to have nostrils as in Platypeltis and Aspidonectes 
of Agassiz. 
As we have only two adult stuffed and four or five young speci- 
mens and no osteological preparations of the American species in the 
British Museum, I shall not attempt to make any observations of 
them, but refer the reader to the work of Professor Agassiz before 
referred to, and proceed to examine with care the specimens of the 
Asiatic and African species in the Museum collection. 
In the Museum Catalogue I showed that the coloration of the 
young specimens, especially the disposition of the colour on the head, 
afforded very good specific character for a certain number of Asiatic 
and African species. But Professor Agassiz, in his account of the 
North American species, shows most distinctly that, though all the 
species of Trionyches, or Mud-Tortoises, found in that country have 
a very similar distribution of colour on the head and shield, yet, 
when the skulls of these animals from different localities are examined, 
they prove to be very distinct—so distinct that he divides them into 
three genera. 
These observations will furnish an example showing how every 
zoologist must be hampered at every step in his progress by the 
limited quantity of the materials at his disposal. I have now, and 
had when I printed my ‘Catalogue of Shield Reptiles,’ every wish 
to examine and arrange the species of this family according to their 
