Dr. J. E. Gray on a new Form of Trionychidse. 205 



Tetrathyra. 



The face of the skull short, convex, arched in front ; orbits lateral, 

 shelving, about midway between the end of the nose and the front 

 of the zygomatic arch ; forehead flat, rhombic, broad. The dorsal 

 shield with flexible margins, without any marginal bones ; front of 

 dorsal shield warty above and without any odd nuchal bone. Ster- 

 num flat, with broad rounded lobes covering the feet, and two pairs 

 of sternal callosities ; the front pair small, rounded, on the front 

 ends of each of the front pair of sternal bones ; the lateral pairs 

 are large, oblong, broadly notched out behind, and very rugose. 



This genus differs from Cyclanosteus in the want of any odd bone 

 in front of the dorsal shield, as well as in the number and disposition 

 of the sternal callosities. 



The upper surface of the front of the disk is closely covered with 

 roundish warts. The sternal callosities are not developed in the 

 young specimen, the larger lateral pair being first indicated as the 

 animal increases in size. The dorsal disk of the young specimen is 

 marked with close grains, or warty, in rather arched longitudinal ridges. 



Lower surface of Tetrathyra Baikii. 



There are some young specimens in spirits from West Africa in the 

 Museum, which belong to this species ; they differ from the young 

 of C. senegalensis in being marbled, while that species is marked 

 with distinct small subcircular black spots. 



This second genus of Cyclanosteina may explain the reason why 

 we have two skulls from West Africa the one with the front and 

 the other with the whole upper edge of the lower jaw dilated, as 

 figured in the 'Proceedings of the Zoological Society' for 1864, 

 fig. 18, p. 95, and fig. 21, p. 96. 



Tetrathyra Baikii, sp. nov. 



Head olive, white-spotted. Back olive, marbled with black above ; 



