236 



Prof. T. H. Huxley. On 



[Mar. 31; 



2. The upper surface of the cranial shield is, at most, rugose in 

 the recent Chelydridce ; in the fossil, three strong conical processes, 

 like horn-cores, of which the middle is the longest, are developed 

 from its posterior and lateral region.* 



This skull is described and figured in the * Philosophical Trans- 

 actions ' for 1886 (Plate 30, fig. 1) by Sir R. Owen, nnder the 

 generic or subgeneric name of Meiolania, and is said to belong to a 

 Saurian reptile closely allied to the " Megalania prisca" described in 

 earlier communications. But the skull is assuredly that of the 

 Chelydroid Chelonian to which the pelvis and caudal vertebra belong. 

 What Megalania jprisca may be I do not pretend to say; but the 

 remains which I have described can have nothing to do with any 

 Saurian reptiles ; and I propose to confer on the genus of Chelonia to 

 which they belong the name of Geratochelys. 



The singular osseous caudal sheaths described by Sir R. Owen, in 

 the same memoir, also appertain to Geratochelys. They formed part 

 of the series of remains sent to the British Museum along with the 

 foregoing, in which none but Chelonian bones have yet been 

 discovered ; and the remains of vertebrae left in these sheaths are 

 similar to the caudal vertebras of the terminal fourth of the tail in the 

 Ghelydridw. The Snapping turtles are noted for the length and 

 strength of the tail and for the strong, laterally compressed, 

 acuminated " scales " which form a crest along the median dorsal line, 

 while others, less strongly keeled, lie at the sides of the tail. In many 

 Chelonia, the extremity of the tail is enveloped in a continuous sheath. 

 These and other scale-like structures in the Chelonia, are usually 

 spoken of as if they were entirely epidermal. But, a day or two ago, 

 Dr. Giinther informed me that in the Australian Tortoise, Manouria, 

 the great imbricated scales of the limbs contain bony scutes ; and that 

 similar scutes are to be found in Testudo grceca. This of course, 

 suggested the examination of the caudal scales of Ohelydra and 

 Gypochelys ; and, having been enabled by Dr. Giinther 's kindness to 

 examine the caudal scales of a good sized specimen of the latter, I have 

 found that those of the crest contain bony scutes. f The bony scute 

 corresponds very closely in form with the whole "scale," but the 

 recurved apex of the latter is formed only by epidermal substance 

 (figs. 5 and 6). 



The living Ghelydra, therefore, has a caudal armature which, in 



* It is possible that these may be dermal bones coherent with the proper cranial 

 shield. 



f The fact is noted by Rutimeyer (Lang and Rtltimeyer, "Die Fossilen 

 Schildkroten von Solothurn," ' Denkschriften der Allg. Schweiz. G-esellschaft,' 

 vol. 22). The armature of the tail in Platysternum is for the most part arranged 

 in zones, of four plates in each zone j but I have not yet been able to find any 

 bone in them. t 



