224 SIRENIA 



genus, which may be identical with Trachytherium of the French 

 Miocene, better preserved remains have subsequently been described 

 by Delfortrie. These show that the rostrum is more elongated 

 than in Halitherium, but the skull is otherwise very similar, as are 

 the molar teeth. The incisors are very large, exserted, strongly 

 compressed, almost sabre -like, rounded on the upper or anterior 

 surface, sharp below, concave on the external and convex on the 

 inner side, and transversely striated. 



Pachyacanthus from the Miocene of the Vienna basin is also, ac- 

 cording to Van Beneden, another form of Sirenian, of which, however, 

 the skull is not known. In various Miocene marine formations of 

 the United States of America other remains of Sirenians have 

 been found, but mostly in such a fragmentary condition that they 

 afford at present little evidence of the .early history of the group 

 in that country. A more satisfactory discovery is that of a 

 nearly complete skull and some bones from a Tertiary limestone 

 formation in Jamaica. It is of smaller size than the Manatee, 

 and, so far as the teeth are concerned, of a still more generalised 

 character than Halitherium, the dentition being apparently i f, c ^, 



/ 1 8\ 



p + m ( jg j = 48. The incisors are small, not developed into tusks ; 



the canines (wanting in all existing Sirenians) are rather larger 

 than the incisors, judging by the sockets ; and the molars are 

 bilophodont, and covered with enamel. It has been described 

 by Sir E. Owen under the name of Prorastomus sirenoides. Some 

 writers regard this genus as the type of a distinct family the 

 Prorastomatidce. Unfortunately we have no knowledge of the geo- 

 logical antiquity of the formation in which it was embedded. Lastly 

 must be mentioned the Eotherium egyptiacum, Owen, founded on the 

 cast of a brain, with a small quantity of surrounding bone, discovered 

 in the nummulitic limestone of Eocene age in the Mokattam Hills, 

 near Cairo. The brain is narrower than in Manatus, and resembles 

 that of Halitherium. This is of interest as the most ancient known 

 evidence of any Sirenian whose age has been geologically deter- 

 mined. Teeth from the same deposits referred to Manatus not 

 improbably belong really to Eotherium. 



The few facts as yet collected relating to the former history of 

 the Sirenia leave us as much in the dark as to the origin and 

 affinities of this peculiar group of animals as we were when we only 

 knew the living members. They lend no countenance to their 

 association with the Cetacea, and on the other hand their supposed 

 affinity with the Ungulata, so much favoured by modern zoologists, 

 receives no very material support from them. 



Bibliography of Sirenia. J. F. Brandt, Symbolce Sirenologicae, St. Petersburg, 

 3 fasciculi, 1846-61-68 an exhaustive account of the anatomy, affinities, and 

 literature of the group, with copious illustrations of the osteology of Rhytina. 



