550 SIRENIA. 



common cutaneous covering, and may or may not have traces 

 of nails. They possess the usual number of phalanges found in 

 mammals. There are no hind limbs. The tail is horizontally 

 expanded into a caudal fin, and there is no dorsal fin. The 

 mammae are pectoral in position close behind the anterior 

 limbs. The skin is thick, naked or with fine hairs scattered 

 over it, and in the deeper layers there is a considerable quantity 

 of blubber for which and for the skin these animals are much 

 sought after. The neck is extremely short, and is hardly 

 marked externally. 



As already hinted, the skeleton is remarkable for its density. 

 In the skuU the anterior narial apertures are placed far back 

 and look upwards, and the nasal bones are usually absent in 

 living forms, though present in some extinct species, but there 

 the resemblance to the skull of the Cetacea ends. The parietals 

 meet in the roof ; the tympanic is annular and ankylosed with 

 the periotic, and readily comes away with the latter ; the cranial 

 cavity is small and elongated ; the zygoma is very stout ; the 

 orbit is small and nearly enclosed in bone ; the maxillae and 

 premaxillae are prolonged in front of the nares as a narrow 

 snout, which carries on its under surface a horned plate 

 working against a similar horny plate on the mandibular sym- 

 physis ; there is a large coronoid process. The dentition is 

 variable. In Rhytina there are no teeth ; in Halicore the molars 

 (^^) are rootless and without enamel, and there are two 



\5 or D' 



tusk-like incisors in the male ; in Manatus, there are no visible 

 incisors and the molars are more numerous (yj) and provided 

 with enamel and roots. The molars are never aU visible at the 

 same time, the posterior coming up as the anterior are worn 

 away. There are milk predecessors in Halitherium, and in living 

 forms there appear to be milk teeth in the foetus (see Kiiken- 

 thai, op. cit.). The centra of the vertebrae are without epi- 

 physes (in living forms) ; the cervical vertebrae (six only in 

 Manatus) are compressed antero-posteriorly, but except the 

 second and third in Manatus are not united. All the vertebrae 

 have articulating processes, though these are imperfect in the 

 caudal region, thus conferring considerable flexibility on the 

 tail. None of them are united to form a sacrum. The ribs are 

 mostly two-headed and the sternum is much reduced. Clavicles 

 are absent. The scapula is normal and not like that of Cetacea. 



