264 SUBUNGULATA order ix 



Cetacea. The structure of the skeleton and their entire organisation, however, 

 leave very little resemblance to the whales to be recognised, and indicate 

 affinity with the ungulates. Skull, teeth and pelvis like those of Moerithermm. 

 The affinity with the Proboscidea suspected by Blainville is now established 

 with certainty. 



In contrast to the light spongy bones of the Cetaceans, the skeleton of 

 the Sirenians is unusually massive in structure, the very dense ribs and 

 vertebrae being especially characterised by a compact, almost ivory - like 

 quality and by considerable weight. The bones of the skull are also in part 

 very dense and the long bones of the extremities are without medullary 

 cavities. 



The very short, disk-shaped centra of the cervical vertebrae usually remain 

 distinct. The centra of the dorsal vertebrae exhibit a subtriangular cross- 

 section, are laterally compressed, keeled below, and the spinous processes and 

 zygapophyses are well developed. The number of lumbar vertebrae is from 

 two to three, that of the sacral vertebrae from one to two. 



The skull (Figs. 351-353) is characterised by strong zygomatic arches, 

 large squamosal bones, and periotic and tympanic of peculiar form. The 

 parietal and frontal bones form the cranial roof ; the nasal bones are 

 reduced, and the narial aperture has moved far backward. The rami 

 of the mandible are deep, robust, and have a high ascending process. The 

 dentition is like that of the ungulates, the lophodont or bunodont cheek teeth 

 fluctuating in number between four and ten. The superior originally 

 sextubercular molars and the inferior quadritubercular molars somewhat 

 resemble those of the tapir and the pig. Absolute distinction between the 

 molars and premolars is never possible, for as a rule no shedding of the teeth 

 occurs. Canines are absent in most of the genera, but sometimes stout 

 incisors are present, especially in the upper jaw. 



The bones of the anterior extremities are flexibly united and movable. 

 A clavicle is wanting. The scapula is sickle-shaped. The radius and ulna 

 are equally long and stout. The carpus originally included three ossicles in 

 the proximal series and four in the distal row, but generally several are 

 co-ossified. The five digits have slender metacarpals and the normal number 

 of phalanges. In the HaUtherium-Halicore series, the pelvis is represented by 

 a long thick ilium and a short broad ischium, but in the manatees the ilium 

 is reduced. In Eotheriam and young JSosiren there still remains a wide 

 obturator foramen between the pubis and ischium. The rudimentary femur, 

 which is concealed in the skin, is merely united with the pelvis by a ligament 

 in later forms. 



The two existing genera, Manatus and Halicore, inhabit the coasts of Africa, 

 India, and Central and South America. Fossil representatives are found from 

 the Eocene onward. 



Prorasfomus Owen. Dentition apparently complete. Incisor teeth circular 

 in cross-section ; superior canine large. Superior molars with two simple 

 transverse ridges directed at right angles to the longitudinal axis of the tooth. 

 Eocene ; Jamaica. P. sirenoides Owen. 



Manatus Rondel. Incisor and canine teeth rudimentary, never functional. 

 From eight to ten cheek teeth above and below on each side, the superior ones 

 three rooted with two transverse ridges and basal cingulum, the inferior ones 

 narrower, with two ridges and talon. In the embryo, traces of three incisors, 



