ADAPTIVE MODIFICATIONS OF LIMB SKELETON 409 



bones in the epipodial series are not misplaced carpals and tar- 

 sals but new growths like supernumerary digits. This idea, 

 however, is not entirely a new one, for Baur himself in the 

 above-mentioned paper refers to it as ** the pisiform or another 

 element of a new-formed ray." Until we know more of this 

 region of the limb, however, it will be impossible for us to de- 

 termine whether the bone in question is a shifted mesopodial, as 

 it may well have come to be during the concentration of this 

 region, or an entirely new structure formed iii situ perhaps by 

 being split off from a neighboring bone. In the meantime it 

 seems well to retain the name "pisiform" already in use and 

 take it with the necessary grain of salt. 



The shifted pisiform occurs only among the latest Ichthy- 

 osaurs [Baptanodoii and OplitJialviosaiims) and Plesiosaurs 

 (^Cimolios aunts). The most advanced condition noticed is that 

 seen in Cimoliosauriis tyocJianteriis where the pisiform lies partly 

 alongside of the distal end of the humerus. The massing of the 

 carpals and tarsals is best know in the Ichthyosaurs, where the 

 bases of the digits are usually also massed, though it is shown 

 equally well by some Plesiosaurs, e. g., Peloneiistcs. 



In some forms the same result of shortening may be attained 

 by the reduction of the carpal and tarsal region, either in num- 

 ber or size of the bones or both. (Cf Dclpliiiiapteriis, PJiocccna, 

 Balcenoptera, etc., among Cetacea, and Tylosaiirus among the 

 Mosasaurs). 



Cartilaginous Progression in Joints. 



Later forms always show the presence of more cartilage than 

 their predecessors, and it is perhaps safe to say that the adapta- 

 tion of a species to aquatic life may be measured by the amount 

 of cartilage developed in the limb joints. The bones of the 

 digits are usually merely separated by cartilage pads — longer 

 near the tip where greater flexibility is desirable — but occa- 

 sionally some of the phalanges may be entirely embedded. 

 The carpal and tarsal bones m all groups tend to become em- 

 bedded in cartilage, with excavated edges, set off from each 

 other, reduced in shape to round plates or ossicles, and in some 



