SIRENIA. 275 



complete skeletons from superficial deposits near its old haunts 

 on the Asiatic coast of Behring Straits. 



Prorastomus. This genus is as yet known with certainty only by a 

 unique skull 0'27 m. in length from an early Tertiary limestone in Jamaica 

 (Prorastomua sirenoides of Owen). The skull is comparatively elongated, 

 and the premaxillary region does not curve downwards over the symphysial 

 end of the mandible, but is more normally disposed, while the cleft of the 

 mouth is straight. The premaxillae, however, are excessively robust, and 

 the external narial opening is in a horizontal plane, though relatively 

 shorter than usual in Sireuians. The teeth are rooted and enamelled, and 

 the series is complete, the dental formula being apparently as follows : 



i. 3, c. 1, pm. 5, in. 3 . 



. The canine is larger than the incisors, and there is 

 i. 3, c. 1, pm. 5, m: 3 



a short diastema behind this tooth above and below. Of the eight 

 molariform teeth in continuous series, the foremost five are termed 

 premolars on account of their comparatively small size and simple 

 structure ; they are tubercular, all with single or only partially divided 

 roots. The three true molars seem to have been bilophodont, with two 

 outer and two inner roots, at least in the upper jaw. Remains of the 

 skull and molar teeth, wanting the rostral end, from the Upper Eocene 

 of northern Italy, have also been provisionally assigned to Prorastomus 

 (P. veronense) ; and one tooth ("fourth upper niilk-inolar ") ascribed to 

 this species exhibits a crown much like that observed in the brachyodont 

 selenodont molars of certain artiodactyl ungulates (e.g. Merycopotamus). 



Halitherium (fig. 159). An early Middle Tertiary genus known by 

 the nearly complete skeleton, which attains a length of about three 

 metres. The skull is typically Sirenian in form, with the downwardly 

 turned premaxillary region, but the nasal bones are well-developed and 

 meet in a suture in the longitudinal median line. The teeth are all 

 rooted, and their crowns are enamelled. There is one pair of tusk-like, 

 though not persistently growing, upper incisors ; but the other front teeth 

 seem to have been rudimentary and are not satisfactorily known. The 

 molariform teeth are in continuous series, and seven in number in each 

 jaw. Of these the first three in the adult are comparatively small and 

 simple, evidently premolars, each with a single root; the fourth tooth, 

 or pm. 4, is also relatively small, but with a double root at least in the 

 lower jaw ; and there is good reason to believe that some were preceded by 

 milk-molars, though the latter have not yet been found actually in posi- 

 tion. The three molars are irregularly tuberculated and ridged, those of 

 the upper jaw with three roots, those of the lower jaw with two roots. 

 The vertebral centra of immature individuals exhibit distinct epiphyses. 

 There are seven cervical vertebrae, all free, except an occasional fusion of 

 the second and third ; nineteen dorsal vertebrae bear the usual robust ribs ; 

 next follow three lumbar vertebra}, of which the third has especially stout 



182 



