220 SIRENIA 



bones. Of these the American Manatee may be known as M. 

 americanus, although it has been described under the names of 

 M. latirostris, and M. australis. The African Manatee (M. senegalensis) 

 differs from the American species in the following cranial characters : 

 the anterior part of the rostrum is shorter, shallower, and altogether 

 smaller ; the orbit is smaller ; the zygomatic process is more deep 

 and massive ; the jugal bone is deeper from above downwards ; the 

 upper margin of the anterior nares is narrower and with a smooth 

 and rounded, instead of a thin and serrated, edge ; the upper surface 

 of the frontal is flat, instead of concave ; the foramen magnum and 

 occipital condyles are narrower from side to side, and the symphysis 

 of the mandible is smaller and shallower. 



Finally, M. inunguis is a fluviatile species confined to the 

 Amazon and Orinoco, which has been but recently fully brought 

 under the notice of zoologists. 



Family HALICORID^E. 



Halicore. 1 In the upper jaw a pair of large, nearly straight, tusk- 

 like incisors, directed . downwards and forwards, partially coated 

 with enamel. In the male they have persistent pulps, and bevelled 

 cutting edges, which project a short distance from the mouth, but 

 in the female, though they remain through life in the alveolar 

 cavity, they are not exserted, and, the pulp-cavity being filled with 

 osteodentine, they soon cease to grow (as in the female Narwhal). 

 In the young there is also a second small deciduous incisor on 

 each side above. At this age there are also beneath the horny plate 

 which covers the anterior portion of the mandible four pairs of 

 slender conical teeth lodged in wide alveolar depressions ; these 

 become absorbed before the animal reaches maturity. The molars 

 are usually f , sometimes f, altogether, but not all in place at once, 

 as the first falls before the -last rises above the gum ; they are more 

 or less nearly cylindrical in section (except the last, which is com- 

 pressed and grooved laterally), without distinction into crown and 

 root, increasing in size from before backwards, with persistent pulps 

 and no enamel. The summits of the crowns are tuberculated before 

 wearing, afterwards flattened or slightly concave. Skull with rostrum 

 formed by the union of the premaxillae in front of the narial 

 aperture, longer than the aperture itself, bending downwards at a 

 right angle with the basi-cranial axis, and enclosing the sockets 

 of the large incisor tusks. Anterior part of the lower jaw bent 

 down in a corresponding manner. Vertebrae : C 7, D 18-19, L and 

 C 30. Tail broadly notched in the middle line, and with two 

 pointed lateral lobes. No nails on the fore limbs. Caecum single. 



1 Illiger, Prodromus Syst. Mamm. et Avium, p. 140 (1811). 



