PEOF. OWEN ON PEOEASTOMUS SIEENOIDES. 561 



anterior and close to the antorbital plate of the maxillary ; but this 

 does not incline forward and outward in advance of the foramen as 

 in other Sirenians recent and extinct. 



I can speak now with more confidence of the extension of the prse- 

 maxillary (PI. XXYIII. fig. 1, 22 ) by a narrow plate, along the upper 

 and lateral border of the horizontal nostril (fig, 2, ri) to the frontal 

 (11) ; but the fractured upper part of the skull still leaves the condition 

 or existence of true nasal bones in Prorastomus uncertain. 



The condyle of the mandible (29) is transversely extended, is 

 slightly convex in that direction, and more convex in the shorter 

 antero-posterior diameter. Erom the outer angle of the condyle the 

 bone descends, more vertically and with less curvature than in 

 Manatus, to the hind border of the ascending ramus. The outer 

 end of the condyle overhangs the outer surface of the ascending 

 ramus, but is less thick than in Manatus. In both rami the con- 

 dyle, with a small part of the hind border of the ascending ramus, 

 has been separated by fracture from the rest of the ramus and 

 cemented in this dislocated state to the inner surface of the ascend- 

 ing branch anterior and external to it. The angle of the jaw has 

 descended below the level of the horizontal ramus ; but to what ex- 

 tent, its fractured state prevents a determination. Nearly 2\ inches 

 of the vertical extent of the ascending ramus is preserved on the 

 right side : the hind border was bent feebly outward as in Manatus ; 

 the lower border, on leaving the rounded angle, is narrower than in 

 Manatus, but thickens as it advances, yet to a less degree than in 

 Manatus. The vertical extent of the horizontal ramus at the alveolus 

 of the last molar is 1 inch 8 lines ; the extreme thickness there is 

 9 lines. The lower border of the ramus describes a feeble concave 

 curve from the angle to the symphysis. The fracture of the lower 

 surface of the symphysis, 5 lines in advance of the hind border, in- 

 dicates some projection, probably like that which characterizes the 

 same part in Manatus. The outlet of the dental canal (PL XXIX. 

 fig. 4, s) is large ; it is an inch in advance of the hind part of the 

 symphysis, 2 inches behind the fore part, and widens where it im- 

 presses the surface of the bone, upon which the impression indicates 

 the ramification of the nerve and vessel, as in. Manatus, Ealicore, and 

 Felsinoiherium. 



There are indications in the left prasmaxillary (PI. XXVIII. 

 fig. 1, 22) of three alveoli, one opening upon the obtuse fore end, the 

 other two upon the contiguous lateral part of the alveolar border : 

 but there is no trace of teeth in these ; they had probably fallen 

 out before fossilization. A the praam axillo-maxillary suture is an 

 indication of the socket of a canine. A diastema, with a feebly concave 

 thin border, intervenes for an extent of about 1 inch 3 lines between 

 the suture or socket of the canine and the foremost upper premolar. 

 This tooth, on the right side, is represented by a long fang, inclined 

 from above downward and backward, with a longitudinal slight 

 indent, indicative of a division of the fang. The position of the roots 

 of two simple-rooted premolars in the lower jaw, which rise in 

 advance of the foremost in place above, shows that this upper tooth 



2p2 



