1. lEioNvx. 243 



character as one of generic importance ; a view iu whicli ho was 

 followed by Gray and more lately by Hcude. As examples of two 

 extreme forms of jaws, the figures given by Gray of 'ryrse nilotlca 

 and Fordid afrlcana (P. Z. S. 1873) may bo 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 having sharp- 

 edged jaws. Heude is precise in his statement to that etfect ; Grav, 

 on the contrary, believes tliat the difference of form and structure 

 above mentioned is permanent, and found in young as well as old 

 specimens (Suppl. Cat, 8h, Hept. 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-lilvc alveolar surfaces convinces n^.o tliat 

 his remarks could not have applied to specimens with the latter type 

 of jaws. 



Having myself found in three species, viz, T. triungiils (Africa), 

 T. cart'daijineus (E, Indies), and T. sinensis (China), examples of 

 the two types, i. e. 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 choanal, in 

 specimens which, in other respects, are uudi.stinguishablo, 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 tho 

 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 difierent from that which has guided Father Heude in his 

 researches. As regards myself, with the cA-idence available at 



