1873.] DR. J. E. GRAY ON THE TRIONYCHIDjE. 45 



2. NlLSSONIA. 



Skull elongate, tapering on the sides in front, the forehead sud- 

 denly bent down ; the nose of the skull between the orbit and the 

 oblong four-sided erect nose-hole not more than half the diameter of the 

 orbits. The front of the palate concave, with a narrow deep groove 

 to the septum, between the internal nostrils, which is rather wider 

 in front, and then about the same width behind, where it is very 

 deep ; alveolar surface very wide, gradually tapering off towards the 

 front of the mouth ; lower jaw very strong ; alveolar surface much 

 wider in front than at the sides, with a deep, short, longitudinal pit 

 in the front half of the front edge, which is rather concave. Hinder 

 part narrower, concave, with a strong prominence on the inner edge. 



Nilssonia, Gray, Ann. & Mag. N. Hist. 1872, x. p. 332. 



The skull from which this genus is described was received in 

 1865, probably from India; but I have not been able to find any 

 Indian Mud-tortoise with which it could be identified. 



The skull is three inches long from the end of the nose to the 

 posterior condyle, and an inch longer to the end of the central lon- 

 gitudinal ridge, and is two inches wide just in front of the tympanic 

 aperture, which is the widest part of the skull. 



1. Nilssonia Formosa. (Fig. 2, p. 46.) 



Only young animal known. Back olive, with four large spots, 

 with a black eye and a narrow white edge. Head with a spot be- 

 hind each eye and at the angle of the mouth, and a large white 

 transverse band on each side of the back of the head, interrupted in 

 the middle of the upper part. 



Skull shorter and broader than that of the adult. 



Trionyx formosus, Gray, P.Z.S. 1869, p. 217, t. 15. fig. 1 (young); 

 Suppl. Cat. Sh. Rept. p. 99. 



Hab. India (Pegu, Theobald's coll.). B. M. 



It appears that this and the other Trionyx marked "Pegu" do 

 not really come from that place ; for although the collection was 

 sold as from " Pegu," it contained many specimens from other parts 

 of Hindostan. 



The skull of the very young animal described as Trionyx formosus 

 (only | inch long) in the British Museum, which has as yet no dorsal 

 or ventral callosities, is very like the adult skull above described, but 

 is shorter and broader, and the groove in front of the internal nostrils 

 is deeper. I think that this is probably the effect of age, and that 

 the skull becomes longer by growth. 



I have named this genus after my old friend Dr. Sven Nilsson, of 

 Lund, who has been working on zoology since 1816, and more lately 

 on archaeology, and is now, in his 86th year, in the full vigour of his 

 intellect. 



Two young specimens in spirits have no sternal callosities ; but all 

 the bones of the sternum are seen through the skin. 



The back of the young is marbled, and has the four eyed spots 

 like the young of the genus Trionyx. 



