OECELLA. . 363 



duly weighed in conjunction with the perhaps otherwise unimportant differences 

 manifested by the skulls, lead to the conclusion that the modification of structure 

 has proceeded to that degree that in its totality it confers a specific character 

 to this form — a resuli which was to be looked for in view of its peculiar habits 

 of life. 



The first point of difference between these skulls, in fully adult individuals, 

 which strikes the observer is the more flattened occipital region of O. fluminalis, 

 directed downwards and backwards, whilst in 0. hrevirostris it arches backwards, 

 downwards, and sKghtly forwards. Another difference consists in the more de- 

 pressed character of the super-occipital and interparietal region of the skull of 

 Orcella fluminalis, an area which is considerably arched in O. hrevirostris. This 

 feature, however, is more pronounced in the skull figured by Owen than in fully 

 adult skulls, and is doubtless a character of immaturity. The next distinctive 

 feature is the shorter and broader snout of the former as compared with the latter, 

 the specific name of which is inappropriate, seeing that it corresponds to no external 

 character, and might therefore mislead to the supposition that the dolphin had 

 a projecting snout in the flesh, of which there is no trace whatever. In Orcella 

 hrevirostris the snout tapers gradually to a point from a slight lateral bulging 

 in the maxillaries, beginning in the adult skull about O'SS inch anterior to the 

 preorbital notch, but in 0. fluminalis the lateral margin tapers to a point much 

 more gradually than in 0. hrevirostris. The maxillaries, too, in the snout of 

 the former, are comparatively flat, and do not shelve downwards on the sides 

 as in the latter species. In 0. hrevirostris the premaxillaries contract opposite to 

 the preorbital notch, while no such marked contraction occurs in O. fluminalis, 

 in which these bones, instead of varying in their breadth, as in the former species, 

 preserve a nearly uniform width throughout. The intermaxillary space is also 

 much wider in O. fluminalis than in 0. hrevirostris, and the nasals are more 

 anterior and lower in the former than in the latter. Viewing the skulls in front, 

 the breadth, on a level with the posterior extremities of the premaxillaries, is 

 proportionally slightly greater in O. fluminalis than in 0. hrevirostris. 



Turning to the base of the skull (fig. 3), we find that the relative size and form 

 of the palatines are quite different in the two species. In 0. hrevirostris these bones, 

 as figured in Owen's drawing of a rather young skull, form a very small portion, 

 viz., 0-16, of the posterior extremity of the mesial line of the palate, and in other 

 skulls of this species before me they form even a still smaller surface. In an 

 adolescent skull they are 0*16, but they do not form a suture, while in an adult 

 cranium they are nearly on a level with the posterior extremity of the palatine 

 surface of the maxillaries. In O. fluminalis, on the other hand, the sutures of the 

 palatine, in an adolescent skull, are well defined, and the palatine is 0*52 inch in 

 length, while in the adult it is 0-60 long and retains the same character. As is seen 

 in Owen's figure of 0. hrevirostris, the tendency of the palatines is to contract in 

 the middle towards the mesial Line, and in two skulls of this species, young and 

 adolescent, the contraction has been carried to complete division, while in the adult 



