AQUATIC MAMMALS 



change in an unpredictable manner. And, this, apparently is just what 

 has transpired in the case of this zeuglodont. 



A change which was fundamentally of this sort has doubtless oc- 

 curred in some of the Cetacea, and in these there must be virtual if not 

 complete elimination of the original ilium, as well as the pubis, but in 

 other sorts there may be an entirely different situation, which the existing 

 pelvis is too greatly altered to indicate with any clarity. 



According to Kellogg (1928) no zeuglodont is yet known having 

 more of a hind limb than a rudimentary femur. In no member of the 

 Odontoceti except the cachalot (Physeter) has any sort of a femoral ru- 

 diment been found, but vestiges, in varying degree, of the pelvic limb 

 have been reported from most if not all species of Mysticeti examined, 

 although the frequency which which it has been stated as entirely lack- 

 ing in certain specimens leads one to suspect that this may be an indi- 

 vidually variable character in some sorts. At least in Balaenoptera phy- 

 salus it apparently is never more than very rudimentary indeed, and Abel 

 (1908) illustrated this detail in a number of individuals, mostly col- 

 lected from the literature. Struthers (1887-88) reported that the hump- 

 back (Megaptera) has a cartilaginous femur inclosed in fibrous tissue, 

 and indicated that he considered that the pubis constituted the greater 

 part of the pelvis. Eschricht also reported a femur in this genus. 

 Flower found that in the blue whale (Sibbaldus) it occurred as a bony 

 nodule, which Beddard stated was attached to the pelvis by one anterior 

 and two posterior ligaments in which there were a few muscle fibers. It 

 is in the balaenid whales, however, that the cetacean limb is least re- 

 duced, and several authors have reported both a bony femur and a car- 

 tilaginous tibial head present in the Greenland or bowhead whale 

 (Balaena mysticetus) . The former was stated by Beddard to be from 4 to 

 9 inches in length. According to Struthers (1880-81) the femur is 

 flexed forward and the tibial rudiment is horizontal, while there is a 

 synovial bursa between the femoral head and the pelvis, and another 

 between the femur and tibia. He (1893) has also stated that there 

 are three muscle slips from the femur to the pelvis. 



That the loss of the external hind limbs in the Cetacea occurred at a 

 rather remote time is indicated by embryologic conditions. Guldberg 

 and Nansen (1894) found that in a 17 mm. fetus of the porpoise 

 Phocaena the pelvic buds were one-third mm. in length, while in one 

 of 7 mm. these were three-quarters mm. long (the fore limbs being one 

 and one-half mm.) and shaped like an oval leaf. Histologically they 

 consisted of undifferentiated mesoderm without sharp separation from 



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