THE PELVIC LIMB 



area may be processes recently and secondarily developed for some 

 specialized purpose, and in others an anterior and a posterior process may 

 actually be remnants of both ilium and ischium, as suggested by the 

 skeleton of a young sperm whale in the National Museum. In this there 

 are two small pelvic bones on each side, one before the other, which 

 one would expect to have fused at a later age. Again, in those whales 

 in which the pelvis is broader at the anterior end there has probably 

 been virtual elimination of the ilium as there seems to have been in 

 Basilosaurus. 



In shape the cetacean pelvis varies greatly. In what seems to be its 

 simplest condition it takes the form of a short, practically straight, 

 slightly flattened bone (in many porpoises) . Or it may be bent some- 

 what after the fashion of a boomerang, but with one end broader than 

 the other (Megaptera), or quite broadly triangular (Sibbaldus), or nar- 

 row at one end and abruptly expanded at the other (some individuals 

 of Balaenoptera, Monodon, etc.) . In a large rorqual it will not exceed a 

 length of 18 inches, and in a small porpoise may measure about one 

 inch. At least those which are more elongate occur practically parallel 

 with the body axis and almost directly above the genito-excretory ori- 

 fice. It has been forced ventrad of its usual situation, evidently by 

 the hypertrophy of the hypaxial musculature and has no direct (bony) 

 connection with the vertebrae. In its simplest form it is doubt- 

 ful whether the cetacean pelvis should really be looked upon as 

 constituting a disappearing remnant of the hind limb, but rather that 

 the hind limb has already disappeared and what remains is merely a 

 necessary anchor for the perineal musculature, thus, possibly as greatly 

 reduced as it will ever become. As these muscles are the more power- 

 ful in the male the pelvic bones average larger in this sex than in the 

 female. 



There has been considerable controversy over the question of whether 

 the cetacean pelvis represents all, or but one, of the elements of the nor- 

 mal innominate. Flower (1876) stated that this bone ossifies from one 

 center and probably represents the ischium. Struthers was of substan- 

 tially the same mind, while Delage (1885) argued that all three bones 

 are represented. Whether the bone actually develops from three centers 

 of ossification seems to me a purely academic question of not much im- 

 port in the present connection. The chief thing is that there has been 

 marked reduction, almost if not quite to the point of elimination, of 

 certain elements of the normal pelvis, and relative increase in the im- 

 portance of at least one other. The important point at issue is whether 

 the present conditions in this regard can be interpreted. 



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