526 CETACEA. 



manner from the external basal line of the crest, being most strongly marked in the 

 constriction; but they disappear in the adult, in which the external surface is 

 coarsely rugose. At all ages, the anterior two-thirds of the inner margins of the crest 

 have a number of imperfections of ossification which communicate with the vacuity 

 between the two plates; these also extend along the anterior border, but do not 

 occur in the posterior third of the margin, as the internal plate is imperfect in that 

 portion of the crest. A number of foramina open behind the anterior border. The 

 anterior and middle thirds of the inner borders of the crests approach each other 

 with age, but never touch ; while their posterior thirds are strongly divergent from 

 within outwards, defining a triangular space and constituting the osseous limits 

 of the blow-hole. The right margin of this space is longer than the left, owing to 

 the sinistral asymmetery of the skull. 



The premaxillary surface is a very narrow area lying along the anterior base 

 of the crest, above the posterior orifice or termination of the cavity of the dental 

 portion of the jaw, and at its posterior half is internal to the fronto-nasal surface. 

 It has an outward and inward curve from above downwards and is completely 

 invested by the premaxilla. The outer margin of the premaxillary portion forms 

 a rather prominent ridge. The fronto-nasal portion narrows from above down- 

 wards between the inner basal margin of the crest and premaxillary surface ; on 

 the right side, it is concave on its upper half, and slightly convex on its lower, while 

 on the left side these characters are reversed. A little below the superior extremity 

 of the premaxilla, and immediately external to its outer border, a foramen occurs in 

 the upper half of the fronto-facial portion and leads nearly vertically downwards to 

 a canal that opens below at the outer border of the palatine, but anterior to it in the 

 upper portion of the spheno-maxillary fossa, defined by the palatine. This canal 

 transmits a nasal branch of the fifth nerve. 



The anterior aspect of the crest, looldng inwards over the nares, consists, in its 

 lower half, of a thin plate of bone corresponding to the flat perforated surface of the 

 external aspect of the maxilla. In this surface, the two plates of the maxilla appear 

 to be lost in each other, where they meet below the orbit, and the external plate of the 

 crest thus appears to be quite distinct from either of them, these amalgamated plates 

 forming the inner plate exclusively. This plate is very thin and is marked by a 

 number of imperfections of ossification in its lower portion; but after passing over 

 the lower half of the anterior or nasal surface of the crest, the imperfections of ossifi- 

 cation become enormous, and the appearance is produced as if the plate stopped short 

 at this point. But, from its anterior portion, a narrower surface is prolonged round 

 the free margins of the crest to near its posterior superior angle ; between this and 

 the body of the plate pass numerous branched osseous spicules connecting the 

 plate and its marginal portion together, the interspaces between the connecting 

 bands being imperfections of ossification. In adults, the inner plate becomes much 

 thickened and consolidated in its lower portion, and the narrow border, along the 

 margin of the crest, also becomes much thickened and detached from the former, 

 by absorption of the osseous connecting bands. 



