THE PECTORAL LIMB 



upon as a splitting into two parts of any particular digit, but as an initial 

 laying-down of six instead of the usual five digital elements. 



Why could not the hyperphalangy of 'the Cetacea be attributable to 

 a similar supernumerary duplication of the phalangeal anlage? This, 

 according to my notion, constitutes a theory that is plausible in more 

 respects than that of either intercalary syndesmosis or epiphyseal differen- 

 tiation, but nevertheless there are particulars which render it unsound. 

 Duplication of the individual phalangeal anlage would not be sufficient 

 to account for it. There would have to be reduplication not once but 

 several times, in the case of the digits of those Cetacea in which hyper- 

 phalangy is most pronounced, of the entire digital complex. And this 

 would have to occur not side by side, as occasionally encountered among 

 Mammalia, but in tandem, so that an entire new digit with three 

 phalanges would be superimposed upon the end of the original digit. 

 But this has never been reported in mammals. If, however, we presume 

 for the sake of argument that such an occurrence be possible, the hy- 

 pothesis can be entertained only until the embryological evidence can 

 be examined. If it were fact then the embryological development of 

 each phalanx would be obliged to progress at approximately the same 

 rate as of all other phalanges. The entire digital complex would have 

 been derived from the three original phalanges, and the ultimate cetacean 

 phalanx would develop at the same rate as the third. But this is not 

 the case. The differentiation of each phalanx is a trifle slower than that 

 of the one next proximad, which effectively disproves the possibility 

 that the supernumerary elements could have been derived by this sort 

 of reduplication. 



(7) Beddard (1910) indicated an opinion that conditions in the 

 Sirenia are more pertinent to hyperphalangy than those in the Otariidae, 

 but he therefrom progressed to a consideration of the Amphibia, which 

 is an entirely different question. 



Both Flower and Lydekker (1876, 1891) have stated that there are 

 never more than the usual number of phalanges in the Sirenia, but some- 

 times in an articulated manus (at least of the dugong) that has been 

 properly cleaned there is to be seen a small, cartilaginous button upon 

 the tip of the ungual phalanx (fig. 35) , as mentioned by Beddard. This 

 I consider to constitute the initiation of hyperphalangy. 



The fact that the digits of the terrestrial Mammalia exhibit a quite 

 remarkable conservatism in that they never develop phalanges in ex- 

 cess of the normal complement is probably partly attributable to the con- 

 servative influence supplied by the almost invariable frequency with 

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