SIRENIA 21.3 



or are only found in a most rudimentary condition, attached to the 

 edge of the frontals, far away from the middle line ; but in some at 

 least of the extinct species these bones, though small in size, are 

 normal in situation and relations. In very few other respects does the 

 skull present any resemblance to that of the Cetacea. In the spinal 

 column of existing forms none of the vertebrae are united together 

 to form a sacrum, and the flat ends of the bodies do not ossify 

 separately, so as to form disc-like epiphyses in the young state, as 

 in nearly all other mammals ; traces of epiphyses have, however, 

 been recently detected in Manatus, and they were fully developed in 

 Halitherium and other fossil forms. The anterior caudal vertebrae 

 have well-developed chevron bones. In one genus (Manatus) there 

 are only six cervical vertebrae. There are no clavicles. The humerus 

 has a small but distinct trochlear articulation at the elbow-joint. 

 The two bones of the forearm are about equally developed, and 

 generally ankylosed together at both extremities. The carpus is 

 short and broad, and the digits five in number, with moderately 

 elongated and flattened phalanges, which are never increased in 

 number beyond the limit usual in the Mammalia. The pelvis is 

 extremely rudimentary, consisting of a pair of bones suspended at 

 some distance from the vertebral column. In no existing species 

 is there any trace of a hind limb, but in the extinct Halitherium 

 an acetabular depression and rudimentary femur have been dis- 

 covered. 



Two kinds of teeth, incisors and molars, separated by a wide 

 interval, are generally present. The former may be developed into 

 tusks in the upper jaw, or may be quite rudimentary. The molars 

 vary much in character. In one genus (Rhytina) no teeth of any 

 kind are present, at least in the adult. Some fossil forms show a 

 more decidedly heterodont dentition, while Halitherium .has milk- 

 teeth, of which no traces have been observed in the recent genera. 

 In all recent types the anterior part of the palate, and a corre- 

 sponding surface on the prolonged symphysis of the lower jaw, are 

 covered with rough horny plates of peculiar structure, which doubt- 

 less assist in mastication. The tongue is small and fixed in position, 

 with a surface resembling that of the plates just spoken of. The 

 salivary glands are largely developed. The stomach is compound, 

 being divided by a valvular constriction into two principal cavities, 

 the first of which is provided with a singular glandular pouch near 

 the cardiac end, and the second usually with a pair of elongated, 

 conical, caecal sacs or diverticula. The intestinal canal is long, and 

 has very muscular walls. There is a caecum, either simple, conical, 

 and with extremely thick walls, as in Halicore, or bifid, as in Maimtus. 

 The heart is broad and flat, with its apex deeply cleft between the 

 ventricles. The principal arteries form very extensive and complex 

 retia mirabilia. The lungs are remarkably long and narrow, as, 



