II56 MANATEES AND DUGONGS 



In the skeleton, the bones are distinguished by their solid and dense structure; 

 this being especially noticeable in those of the skull and in the ribs. The skull is 

 depressed and has a more or less distinctly deflected beak-like snout, much flat- 

 tened from side to side. It is further characterized by the very large size of the 

 aperture of the cavity of the nose, which is somewhat pear-shaped, and placed rela- 

 tively further back than usual. In the living forms the nasal bones were either 

 rudimentary or totally absent; but in some of the fossil species they were better de- 

 veloped, and partially roofed over the nasal cavity in the ordinary manner. This 

 abortion of the nasal bones in both the Sirenians and Cetaceans is doubtless due to 

 the necessity for a large nasal aperture in the skull, owing to the peculiarities in 

 the respiration of these animals. The vertebrse are peculiar in that during the 



young state they do 

 not show separate 

 plate-like ossifications 

 XL each end of their 

 bodies, like those de- 

 veloped in other Mam- 

 mals. Rudiments 

 of these so-called epiph- 

 SKEUSTON OF MANATEE. yscs have, however, 



been shown to exist in 



the extremely young state; and they were more fully developed in certain extinct 

 forms. As in Cetaceans, none of the vertebrse in the hinder region of the trunk 

 unite to form a sacrum; and it is evident that a solid immovable structure in this 

 part of the backbone would be not only a serious disadvantage to a swimming ani- 

 mal, but likewise of no possible use to one which has no hind-limbs to support. 

 Sirenians resemble Ungulates in having no collar bones. In the fore-limb the upper 

 bone, or humerous, is of considerable length, and differs from that of the Cetaceans 

 in having distinct pulley-like surfaces at its lower end for the articulation of the 

 bones of the fore-arm (radius and ulna), thus permitting of a certain amount of free 

 motion at the elbow joint. The two bones of the fore-arm are, however, generally 

 united at the lower end. The number of the digits is five, and none of these con- 

 tain more than the ordinary number of three joints, in addition to the metacarpus. 

 None of the recent Sirenians show any trace of the hind-limb, although the pelvis 

 is represented by a pair of splint-like bones; but in some fossil forms there was a 

 rudimentary thigh bone, or femur. 



There are several peculiarities connected with the soft internal parts; but 

 it will suffice to mention here that the lungs are extremely long and narrow, ex- 

 tending beneath the backbone nearly as far back as the last rib. To permit of 

 this backward extension, the midriff, or diaphragm, is placed very obliquely. 

 The larger arteries of the body form peculiar net-like expansions in certain 

 regions, which render the animals able to remain beneath the surface of the 

 water for a longer period than would otherwise be possible, as partly oxygenated 

 blood can be retained for some time in these structures before it is passed through, 

 the heart. 



