436 ANATOMY OF VERTEBRATES. 



posterior costa is concave, deepest in the Dugong. The outer 

 surface has a spine about half the length of the bone, marking off 

 a broad pre-spinal, from a narrow post-spinal fossa : the spine 

 is produced into a slender acromion in the Manatee, not in the 

 Dugong. 



The humerus, ib. 53, has the normal mammalian character, 

 though of small size, wdth the head, tuberosities, and deltoid crest, 

 the twisted shaft, the epicondyloid processes and intermediate 

 trochlear articular surface, for synovial articulation with the 

 coalesced proximal ends of radius, 54, and ulna, 55. The latter 

 developes an obtuse olecranon : the distal ends of the antibrachial 

 bones are extensively united and ultimately by bone. In the 

 Manatee there are six carpals, three in each row : the outermost 

 and largest represents a cuneo-pisiform, and articulates with both 

 the ulna and the fifth metacarjoal. The trapezium and trapezoides 

 are represented by one bone articulating with the first and second 

 metacarpal : the magnum supports the third, and the unciforme 

 the fourth and part of the fifth metacarpals. In the Dugong 

 there are but three carpals : the scapho-lunar and cuneo-pisiform 

 in the first row, 56, and a single transversely oblong bone repre- 

 senting the second row, but leaving the major part of the base of 

 the fifth metacarpal to articulate mth the cuneiform. The pollex, I, 

 is represented by a styliform metacarpal : the other digits have 

 each three phalanges ; and most of the ungual ones, in Manatus, 

 support nails. All the limb-bones, like those of the rest of the 

 skeleton in Sirenia, are solid. 



The herbivorous Sirenia have not to move far from their 

 favourite localities for food ; they contrast, in that respect, with 

 the Cetacea that pursue a living prey : hence the difference in the 

 specific gravity of the bones, which in Sirenia is such as to re- 

 quire an effort on the part of the animal to reach the surface of 

 the water for breathing, but enables them to browse, at ease, the 

 vegetation clothing the bottoms of their seas, estuaries, or rivers. 

 The massiveness of the zygomatic arches in the skull contrasts 

 singularly with the slenderness of those parts in Whales: the 

 pterygoid productions offer a similar difference : the external 

 bony nostril is as remarkable for expanse in Sirenia as for con- 

 traction in Cetacea. The movements of the head and jaws, in 

 browsing, call for a flexibility of the short neck in Sirenia, in- 

 compatible mth the fixation of that part which prevails in most 

 Cetacea : the dorso-lumbar vertebras are articulated by true zyga- 

 pophyses, not metapophyses. The pleurapophyses are as remark- 

 able for thickness and density in Sirenia, as in similar-sized Cetacea 



