xvii.] SIRENIA. 323 



process of the sacral vertebra, usually the fourth behind the 

 last that bears a free rib ; the lower one is the ischium, or 

 ischium and pubis combined, and approaches, though it 

 does not meet, its fellow of the opposite side. 



In the adult Manatee, the innominate bone is represented 

 by a single irregular triangular bone, connected by rather 

 long ligaments with the vertebral column above, and with 

 the opposite bone across the middle line. 



There is no trace of an acetabulum, or of any portion of 

 the limb proper in any of the existing Sirenia ; but in the 

 extinct Halitherium an acetabular depression and rudimentary 

 styliform femur have been discovered. 



In the CETACEA the pelvis is reduced to a pair of elon- 

 gated slender bones (each of which ossifies from a single 

 ..centre), placed on each side of, and rather below, the ver- 

 tebral column, lying nearly parallel to its long axis (though 

 they converge somewhat anteriorly), and opposite the spot 

 where the chevron bones commence to be developed 

 beneath the bodies of the vertebrae. These bones probably 

 represent the ischia, and their principal function is to 

 give attachment to the crura of the penis or clitoris, as 

 the case may be. Hence they are usually more largely 

 developed in the male than in the female. 



In the Whalebone Whales they usually have a project- 

 ing angle placed about the middle, near which, in some 

 species, a second small bone, which probably represents 

 the femur, is attached by ligament (see Fig. 120, p. 335). 

 In a full-grown male Rorqual (Balcznoptera mwculus), 

 sixty-seven feet in length, each pelvic bone was sixteen 

 inches long. In the Greenland Whale (Baltzna mysticetus,} 

 they are rather shorter and stouter. As might be expected, 

 from the rudimentary character of these bones, they vary 

 considerably in size and form in different individuals of 



Y 2 



