THE PELVIC LIMB 



I think the evidence is clear that there has been reason for retention 

 of a part of the ischium, for perineal attachments, and it may reasonably 

 be conceded that a considerable part of the present pelvis — say at least 

 half — is of such derivation. // another pelvic element has been retained 

 in appreciable amount it probably consists of the ilium, for the muscles 

 usually attached to the pubis have disappeared or relinquished pelvic 

 connection. 



This brings us back to conditions in the Sirenia. It is incontrovertible, 

 I think, that the plan of pelvic reduction in the Trichechidae has been 

 on a basically different principle from that in the Halicoridae. If this 

 can be so in the Sirenia it can apply with equal logic to the Cetacea, and 

 accordingly it must be postulated that in the whales the pelvis may have 

 suffered reduction on at least two, and possibly more, basically different 

 schemes, each perhaps varying in minor details according to the group 

 concerned. Of those conditions involved in a possible retention in 

 definite degree of the ilium nothing can be told, but considerable can be 

 inferred about that plan by which the ilium may have been eliminated, 

 just as in the manati, by analogy to the pinnipeds and zeuglodonts, as 

 follows. 



The pelvis of the zeuglodont Basilosaurus (fig. 51) was considerably 

 less advanced in specialization toward simplification than in any living 

 whale. This is a roughly quadrangular bone with an indubitable acetabu- 

 lar depression upon its extreme anterior part, for attachment of the ru- 

 dimentary femur, and posterior to this, a foramen which is obviously a 

 relic of an obturator foramen. One can hardly escape the conclusion 

 that this situation represents an extreme atrophy of the ilium and the 

 entire disappearance of all but possibly that part forming the anterior 

 margin of the acetabulum. Evidently the zeuglodont ancestor had be- 

 gun to trail the hind limbs and there ensued a reduction in the length of 

 the ilium, just as is now to be seen in the pinnipeds, and further atrophy 

 of the hip muscles arising from this bone and relinquishment of its 

 iliocostalis attachment would naturally be followed, in the course of 

 time, by the entire suppression of the pre-acetabular part of the pelvis. 

 Accompanying the reduction of the muscles attached to the ilium would 

 be a similar atrophy of those of the hip and thigh that were attached to 

 parts of the ischium and pubis. There would naturally follow atrophy 

 of these parts of the innominate bone, but there would not be the same 

 relative degree of atrophy of the ischial portion which afforded attach- 

 ment to the perineal muscles, and these would increase in proportionate 

 importance. As a matter of course the form of the complex would 



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