PLATANISTA. 527 



Sexual differences displayed hy mamillaries. — The characters which distinguisli 

 the snouts of the males and females are illustrated by the skulls represented in Plate 

 XXXIX, and I have already pointed out what these characters are. 



Changes by age in the alveoli and dentition. — The alveolar surf aces lie side by side 

 throughout their extent, except posteriorly, where they are divergent at the fourth 

 tooth from the last. In very young skulls the alveolar furrow posteriorly, as far 

 forwards as the eighth or ninth tooth from the last, is continuous with the cavity of the 

 maxilla, but about these teeth a narrow ridge appears on the inside of its external 

 wall. This ridge soon re-unites with its inner wall, constituting a distinct roof per- 

 forated by foramina, which separate the furrow from the cavity of the jaw, through- 

 out, however, only a very limited part of its extent, as this septum ceases about the 

 twentieth tooth from the hindmost, the jaw cavity and furrow being merged in one. 

 The formation of alveolar pits I have also previously described under Dentition. 



Fremaxilla (PL XXXIX, figs. 1 and 2, and PI. XL, figs. 17 and 18).— This 

 bone is nearly of the same length as the maxilla and consists of two portions, 

 a facio-nasai plate, almost crescentic in form, constituting the outer border of the 

 external nares, and a long thin compressed rod continued forward beyond the extremity 

 of the maxilla. The former plate overlaps the f acio-nasal portion of the maxilla, closing 

 in the posterior termination of the cavity of the jaw and completing the anterior wall 

 of the nasal canal. The remaining portion of the bone is nearly straight in youth, 

 but, in adults, it partakes in the same marked curves that distinguish the maxilla. The 

 outer surface of the premaxilla, about the beginning of the furrow lodging the infra- 

 orbital vessels and nerves, is marked by two or three imperfections of ossification in some 

 skulls, but not in others. The premaxillary foramen occurs about one inch from the 

 end of the snout in adult skuUs, and is close to the upper border which is slightly 

 divergent from its fellow, displaying the termination of the cartilaginous vomer. 

 As already stated, the premaxilla appears to carry four teeth, as the line of suture 

 can be traced along to the base of the fourth tooth ; union, however, of the anterior 

 extremity of the bone with the maxilla is an event of uterine life. The inner 

 surface is divided into two portions, an upper deeply concave portion lodging the 

 mesethmoid cartilage, and a thin plate below this, which gradually increases in depth 

 from before backwards, applied to the inside of the vertical or palatine plate of 

 the maxilla. 



This bone also partakes of the general asymmetry of the skull, its left facio- 

 nasal portion having a considerable sinistral twist, owing to the external nares being 

 dragged to that side of the skull, a circumstance which also tends to reduce the 

 upward extension of this part below the level of the bone of its right side. 



Nasals. — These two small narrow bones lie in a depression on the lower end 

 of the nasal ridge of the frontal, above where it is continuous with the mesethmoid 

 portion of the ridge. The lower extremities form a very faint projection, marking 

 the superior posterior termination of the nasal septum. 



The Mandible and its dentition (PI. XL, fig. 19, and PI. XXXIX, figs. 1 

 and 2).— The distinguishing feature of the mandible, as is well known, is the very 



