361 



CETACEA. 



it is all but complete. In O.fluminalis the tendency to division also exists in the 

 adolescent skull, but to a much less extent, while in the adult there is little or no 

 indication of it. The measurements of these parts may be stated as follows : — 



Sutures of palatines 



Length of palatines obliquely across the palate 



The distance between the bases of the pterygoids where the palatines 

 intervene is greater in O. fluminalis than in the other species, 

 and may be tabulated thus, viz 



O. brevirostris. 



0-16 

 1-24 



0-44 



0-16 

 1-32 



0-52 



O.OO 

 1-24 



0-52 



o.fluminalis. 



052 

 1-80 



0-64 



0-60 

 1-88 



0-64 



The form of the palatines differs somewhat in the two skulls. They are 

 relatively larger in O. fluminalis, and their middle margins are first directed back- 

 wards and inwards and then markedly outwards and backwards, whilst in 0. 

 bremrostris the whole direction from their bases is outwards and backwards. 



I lay considerable stress on the relations and form of the palatines, because 

 the characters I have described are persistent in Orcella bremrostris and in two 

 examples of O. fluminalis, and, moreover, on referring to the skulls of two 

 species of an alHed genus, Globicephalus, viz., G. svineval, Lacep., and G. indicus, 

 Blyth, I find they also are distinguished from each other by well-marked and 

 persistent characters in the form of their palatines. In G. indicus the pterygoids 

 are only separated from each other by a very narrow plate of the palatines, while in 

 G. svineval these bones are very broad and form a large surface widely separating 

 the pterygoids. It is also a significant fact that one of the distinguishing features 

 of G. indicus (G. macrorhynchus) from G. svineval is the relatively longer and 

 narrower snout of the latter as compared with the former— a condition the exact 

 reproduction of that which occurs in these small round-headed dolphins, in wliich the 

 proportionally shorter snout of O.fluminalis has large palatines, w^hile the longer- 

 beaked skull of O. brevirostris has these bones feebly developed. 



Another distinction may be noted, viz., the more marked concavity of the supra- 

 orbital plate of the maxillary of O.fluminalis as contrasted with O. brevirostris. 



The tympanic and periotic are markedly distinct in the two species. In O. 

 fluminalis, the latter (Plate XLIII, fig. 4), when viewed from above, is seen to have 

 a different form from O. bremrostris (fig. 9) in the external half of its internal border, 

 which is prolonged backwards and outwards to the mastoid process in a swollen 

 outline, while the same part in O. brevirostris is concave. Anterior and inter- 

 nal to til is border the periotic is pyramidal in O. fluminalis, wliile in O. bre- 

 virostris it is rounded and broad at its tip, and somewhat contracted at its base, and 

 of much greater size. The tympanic of O. fluminalis is proportionally larger than 

 in O. brevirostris, and slightly more pointed, and the posterior inferior border 

 is flattened, instead of being rounded as in the latter. In O. brevirostris the pos- 

 terior lobe of the tympanic is not so long as in the Irawady dolphin, in which it is 

 not marked by the deep depression which occurs on the posterior surface of this part 

 of the bone of the marine form. 



