ADAPTIVE MODIFICATIONS OF LIMB SKELETON 465 



ably the first and fifth of the original pentadactyl limb, have 

 vanished entirely. 



The elongation of the digits may take place by an increase 

 either in the length or the number of phalanges, — hyperpha- 

 langy. Undoubtedly the former method is the original one as 

 we find it in many forms not otherwise greatly modified. The 

 Pinnipedia, the marine Chelonia and the Thalattosuchia show 

 only this modification. The earlier Mosasaurs show very little 

 hyperphalangy while the phalanges are greatly elongated. 

 The whalebone whales are much less modified in this respect 

 than the toothed whales and in them the elongation is largely 

 due to elongation of the bones, while in the latter group hyper- 

 phalangy coupled with a cartilaginous separation of the bones 

 is the general mode. In the Plesiosauria and Ichthyosauria, 

 and particularly m the latter group, hyperphalangy is devel- 

 oped to such an extent that it accounts entirely for the length 

 of the digits. It is only in such early Ichthyosaurs as Mixo- 

 saurus and Merrianiia that we can observe any characters of 

 long bones present in the phalanges. 



Hyperphalangy. 



Several theories have been proposed to account for hyper- 

 phalangy. Howes has suggested an '' intercalary syndesmosis " 

 such as occurs in some Amphibia, the formation of a joint in the 

 middle of a bone by ossification of the ends of the bone only, 

 the joint appearing in the cartilaginous portion remaining, but 

 there is no substantial evidence to support this theory in the 

 reptiles and mammals. 



Weber, Ryder and Baur have tried to prove that the addi- 

 tional phalanges were formed by the ossification of terminal or 

 extradigital cartilages such as those that appear on the ends of 

 the digits in the Pinnipedia, or to quote Ryder, "The carti- 

 laginous extension of the ungual phalanges, I take it, has 

 afforded the basis, in some ancestral seal-like form, for the 

 development of an increased number of digits beyond the 

 ungual phalanx, as in Cetacea." While there may be nothing 

 in the nature of things to prevent such a process taking place. 



