THE GANGETIC MUB-TUllTLE. 515 



of two small bony spicules, the most external (4, see sketch, p. 516) 

 corresponding to the outer wing of the hyoplastron of Trionyx, 

 and the internal spicule (3) to the forwardly and inwardly project-- 

 ing process of the hyoplastron of that genus of freshwater turtle, 

 to the outside of which the backwardly and outwardly directed 

 ramus of the entoplastron is applied. 



These two bones or elements of the hyoplastron o? Emyda dura, 

 in the two individuals examined, were separated from each other 

 posteriorly, opposite to the hypoplastron, by a cartilaginous 

 interval (*) which permitted of their being freely moved one upon 

 the other. At this point, or their angle of convergence, a mem- 

 branous interval existed between them and the hypoplastron, 

 corresponding to the fold of the embryo. The plastron of this 

 genus, therefore, if these observations are verijfied by further 

 research, and if none of the other elements are of a compound 

 nature, may prove to consist of eleven distinct bones, two of 

 which occupy the position of the hyoplastron of Trionyx. This 

 peculiar character of the plastron of Emyda, if of general occur- 

 rence (which has yet to be ascertained), does not at any rate 

 extend many hours beyond embryonic life — because in recently 

 born individuals I have never observed the hyoplastron in any 

 other condition but that of a single bone which unites at an 

 early age with the hypoplastron, while in 'Urionyx and Clutra 

 the embryos which I have examined have the hyoplastron con- 

 sisting of one piece, which only unites with the hypoplastron at 

 an advanced period of life. 



Whatever may be the explanation of these two instances of a 

 compound hyoplastron in Emyda dura, there can be no doubt of 

 the accuracy of the observation, which was verified by one of my 

 assistants ; but it is of sufficient importance, whatever be its 

 cause, to be recorded and to receive further investigation. 



PS, — Interested in the foregoing fact of development and of 

 its value at issue, I append the subjoined memorandum from a 

 palaeontologist. 



" The condition of the hyosternal bone described in this young 

 specimen by Dr. Anderson is suggestive of several fossil types. 

 In an undescribed genus from the London Clay, known as Emys 

 levis, as well as in Platemys BoiverhanTcii, there are distinct lateral 

 elements in the plastron which occupy the position of the lateral 

 ossifications of Dr. Anderson's specimen, being placed between 

 the hyo- and hypo-sternal elements and the marginal bones. In 



37* 



