THE SIEENIA. 389 



cranial cavity is worthy of notice, as it strongly contrasts 

 with the form of the brain-case in the Cetacea. The supra- 

 occipital is very large and slopes upwards and forwards 

 a long way on to the upper surface of the skull; but it does 

 not separate the parietal bones ; which, as usual, unite in 

 the sagittal suture. The frontals are prolonged into broad 

 supra-orbital processes. The nasal bones ai-e abortive, and, 

 in the dry skiill, the external nares are very wide and look 

 upwards. The tympanic bone is a thick hoop, ankylosed 

 with the periotic bones, and readily comes away from the 

 skull with them. The zygoma is enormously stout. The 

 premaxillss constitute a large portion of the boundary of 

 the gape ; and the lower jaw has a high ascending portion, 

 with a large coronoid process. 



The scapida has a distinct spine occupying the ordinary 

 position. There are no clavicles. The humerus has its 

 distal end fashioned into articular surfaces, vipon which the 

 radius and ulna are freely movable. The pollex is rudi- 

 mentary, and the other digits have no more than three 

 phalanges each. 



The pelvis is rudimentary, the bones which represent 

 the ossa innominata being connected by their proximal 

 ends with the transverse processes of the last of the pre- 

 caudal vertebrae. They are disposed vertically to the axis 

 of the body. No trace of the hind limbs has been obsei-ved 

 in any of the existing Sirenia. 



The premaxillary region of the palate, and the corre- 

 sponding surface of the mandible, are coated with mam- 

 millated and nigose horny plates formed of hardened 

 epithelium ; and, in the extinct genus Rhytina, these plates 

 were the only masticating organs, as there were no teeth. 

 In Halicore (the Dugong), there are teeth which have no 

 vertical successors, form no roots, and are devoid of 

 enamel ; while, in Manatus, there are milk molars, and the 

 grinding teeth are enamelled, and present crowns with 

 double transverse ridges. 



The adult Manatee has no incisors. In the Dugong 

 there are no incisors in the mandible of the adult. The 



