1. TRIONYX. 243 
character as one of generic importance ; a view in which he was 
followed by Gray and more lately by Heude. As examples of two 
extreme forms of jaws, the figures given by Gray of Tyrse nilotica 
and Fordia africana (P. Z. 8. 1873) may be consulted. It is, how- 
ever, to be noticed that none of these authors appear to have met 
with young specimens in which the “molar” character of the 
alveolar surfaces was developed, all young specimens haying sharp- 
edged jaws. Heude is precise in his statement to that effect ; Gray, 
on the contrary, believes that the difference of form and structure 
above mentioned is permanent, and found in young as well as old 
specimens (Suppl. Cat. Sh. Rept. i. p. 87); but the fact that I have 
been unable to find, in the British Museum collection, a single young 
specimen with the molar-like alveolar surfaces convinces me that 
his remarks could not have applied to specimens with the latter type 
of jaws. 
Having myself found in three species, viz. 7’. triunguis (Africa), 
T. cartilagineus (Ki. Indies), and 7’. sinensis (China), examples of 
the two types, 7. ¢. on the one hand sharp-edged, comparatively 
narrow jaws, and on the other hand broad crushing alveolar sur- 
faces nearly meeting on the median line in front of the choanee, in 
specimens which, in other respects, are undistinguishable, I have 
arrived at the conclusion that we may be in presence of a case of 
dimorphism caused by a difference of diet, a view which has been 
strengthened by a perusal of Heude’s notes on the habits of the 
Chinese Trionychoids. 
I fancy that individuals of the same species may, according to 
circumstances, take to two distinct modes of feeding : the carnivorous, 
chiefly ichthyophagous, for which the retention of the sharp beak, 
with which all young Trionychoids are provided, is necessary ; and 
the conchifragous, by which the jaws gradually acquire a great 
increase of width and strength, accompanied by the loss of the 
sharp edge. When once a certain amount of specialization in the 
latter direction has been reached, hunting for active prey becomes 
more and more difficult, and the character hence more and more 
developed as the animal grows older and takes to crushing harder 
and harder objects, such as large shells and hard seeds, as Heude 
has actually observed. Accompanying the more massive mandible, 
a second character makes its appearance, viz. the presence of a 
tubercle on the lower border of the jugal bone, which is correlative 
of the increased development of the temporal muscle which moves 
the lower jaw. 
Although thus separated into two groups, according to their mode 
of obtaining food, the individuals of one and the same species are 
not in any way, it appears to me, prevented from interbreeding, for 
we know that they coexist in the same waters, and therefore the 
characters have no opportunity of becoming fixed and permanent. 
The final solution of the problem rests with those who can 
observe the animals in nature, and, I need hardly add, with a turn 
of mind different from that which has guided Father Heude in his 
researches. As regards myself, with the evidence available at 
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