568 MANATIS AND DUGONGS. 



forms have horny plates on the palate and on the opposing surface of the lower 

 jaw. 



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 flattened 

 from side to side. It is further characterised by the very large size of the aperture 

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

 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 

 developed, 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 peculi- 

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

 the young state they 

 do not show separate 

 plate - like ossifications 

 at each end of their 

 bodies, like those 

 developed in other 

 mammals. Rudiments 

 of these so-called 

 epiphyses have, how- SKELETON OP MANATI. 



ever, 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 vertebrae 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 swim- 

 ming animal, 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 humerus, is of considerable length, and differs from that of the 

 Cetaceans in having distinct pulley-like surfaces at its lower end for the articu- 

 lation 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 contains more than the ordinary number of three joints, in addition 

 to the metacarpus. None of the recent Sirenians shows 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, extending 

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

 ward 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. 



