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 of 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. 
