THE PECTORAL LIMB 



one simple cartilaginous element after another, according to need, upon 

 the tip of the normal digit. I claim no great originality for this. It has 

 been implied, at least partially, in the past, but I know of no one who 

 has really championed it as the method by which hyperphalangy has 

 been developed. 



( 1 and 2)1 believe that the countless anatomical details of the Cetacea 

 that are either fundamentally or precisely similar to those of terrestrial 

 mammals constitute overwhelming evidence that the order is directly 

 derived either from a strictly mammalian stock, or at least from the 

 same. group of mammal-like reptiles from which the land mammals took 

 origin. I hasten to affirm that the latter statement is made purely in 

 the line of argument, and that there is not as yet the slightest reason 

 for believing it to be so. Furthermore, if one believed that the Cetacea 

 were derived directly from amphibia, or "swamp-inhabiting creatures", 

 then anatomical details render it obligatory also to embrace the hypothesis 

 that all mammals had a similar direct ancestry, rather than from ter- 

 restrial, mammal-like reptiles. The possibility that the Cetacea sprang 

 from the ichthyosaurs is utterly untenable, in which opinion every anato- 

 mist will agree. Before progressing to the next question, however, it 

 is in order to examine more thoroughly the quality of the hyperphalangy 

 that occurs in the manus of aquatic reptiles. This, of course, varied in 

 direct degree to the completeness of aquatic adaptation, and relative to 

 other considerations. Thus the flipper of the highly aquatic Geosaurus 

 (fig. 34) had but one more phalanx than the normal mammal, but the 

 single carpal element upon its anterior border, the first metacarpus and 

 first phalanx of the pollex, are enormously more robust than the other 

 comparable ossicles, which probably indicates that the flipper was long 

 used as an efficient oar before it became so reduced in size. In Elasmo- 

 saurus (a plesiosaur) hyperphalangy was about as far advanced as in 

 those whales which show this to best advantage (although other elements 

 of the arm differed considerably) . Some ichthyosaurs had as many 

 as twenty-six phalanges in the third digit, and in these reptiles the bony 

 elements of the entire arm were essentially similar, save for the slightly 

 larger size of the humerus, radius and ulna, and were all closely packed 

 in a bony mosaic. Williston (1925) mentioned that this condition could 

 not be due to ossification and separation of epiphyses, for these reptiles 

 had none of the latter. Whatever was the mode of their development, 

 it is not necessary to infer that this was the same as in the Cetacea. Both 

 groups encountered stimuli for an increase of phalangeal elements and 

 both have exhibited this convergence, but it is by no means certain that 



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