AQUATIC MAMMALS 



erate depths. Why should they be better fitted for doing so than whales, 

 which descend to great depths? Also it would seem of at least equal 

 importance that they should be able to rise to the surface with celerity. 

 As a matter of fact they have doubtless adopted a middle course and 

 their specific gravity is probably almost the same as that of the water 

 which they displace. If this be the case then their bones are particularly 

 dense to compensate for the unusual lightness of the rest of the body. 



As sirenians never leave the water the thoracic stimulus of fore leg 

 support usually present in the terrestrial mammal is entirely lacking, as 

 it is in cetaceans, with this difference, that in sirenians the anterior limb 

 is much more mobile. Appreciable gravitational pull upon the thorax 

 is also lacking, and there remain as discernible influences only the 

 stimulus for an aquatic mammal to assume a circular thoracic cavity, and 

 the effect that levitation by the inflated lungs may have. As a matter of 

 fact in cross section the chest seems to be definitely broader than high. 

 Levitation by the lungs might be expected to raise the curve of the ribs 

 well above the vertebral column, but this is no more marked than the con- 

 dition encountered in many terrestrial mammals, both quadrupedal and 

 bipedal. 



Sirenians have experienced quite a remarkable lengthening of the 

 thorax which has operated to shorten the lumbar region. There seems to 

 be considerable variation in the number of ribs. Stannius recorded a 

 manati with 15 ribs while Murie encountered individuals with 16, 17 

 and 18 pairs. Halicore may have 18 or 19- In the latter genus the con- 

 formation of the ribs is not unusual, but in Trichechus they are very re- 

 markable. In this animal the individual thoracic vertebrae are much 

 longer and therefore the rib centers are farther apart. Presumably in 

 compensation the ribs are phenomenally broadened so that the intercostal 

 spaces are not particularly wide. Furthermore in this genus (at least 

 in the species latirostris) the distal ends of the first twelve ribs are on a 

 line virtually parallel with the vertebral column, the remainder of the 

 series becoming successively shorter. It is said that always in this order 

 all ribs have both capitular and tubercular attachment to the vertebrae. 

 This character increases the rigidity of the thorax and might be expected 

 to accompany a long sternum with strong and well calcified sternal ribs. 

 On the contrary the sternum is much reduced and in the manati espe- 

 cially about two-thirds of the costal series apparently have no costal car- 

 tilages at all, their atrophy leaving a relic in the shape of nodular bony 

 growths upon the distal extremities of the true costae. Flower has il- 

 lustrated the sternum of a young Halicore in which four pairs of costal 

 [166] 



