ADAPTIVE MODIFICATIONS OF LIMB SKELETON 453 



ally and occurs in all groups. In a few forms, by way of excep- 

 tion, the digits are quite straight but in every case these forms 

 seem to be earlier and less adapted to aquatic life, and in every 

 group where straight digits occur it is worthy of note that the 

 species occurring later have the digits curved. Occasionally 

 the limb may be sufficiently curved in some other region, when 

 the digits will be more nearly straight (cf Cimoliosaiwiis eury- 

 tnerus). The Cetacea, without exception so far as my observa- 

 tions have carried, have the fingers more or less curved. Some- 

 times, as in Balceua and BalcBuoptera the curvature may be slight, 

 while in others, c g., Globiocepliahis^ the curvature is very great. 

 The Plesiosaurs, also, seem to have the digits curved in all 

 cases. In Cimoliosaiirus eiirymeriis the curvature is but little ; 

 in Plesiosaunis dolichodeinis it reaches the other extreme. The 

 Thalattosuchia (cf Geosauriis) show the backward curvature, 

 also. In the Mosasaurs, M. leuioinieri seems to have the digits 

 of the hinder liinb perfectly straight, and the hand of Clidastes 

 velox is little, if at all, curved. It is worthy of note here, how- 

 ever, that these forms also show a more primitive condition of 

 carpus and tarsus and other indications of lesser adaptation to 

 aquatic life than is found in Platecarpits and Tylosaiiriis^ for 

 example, which have the digits well curved. The Ichthyosaurs 

 present a variety of conditions. The Triassic Mixosaiirtts, which 

 is undoubtedly the most primitive, has the digits distinctly curved. 

 Some of the later Jurassic forms, such as IcJitliyosaiirtis aciitiros- 

 tris and /. coiiimiinis have the digits quite straight and these 

 species are much more modified in some respects than Mixo- 

 saiirus. However, against this must be placed the fact that in 

 /. quadriscissiis, I. ingens and Baptanodon nutans, which by car- 

 tilaginous separation of the bones and by other signs point to 

 a greater adaptation to a swimming life, have the digits curved. 

 Again the Triassic genera Merrianiia {Leptocheirns) and Toi'e- 

 tocnemus which Merriam has recently described and which are 

 nearly if not quite as old as Mixosanriis, have the limbs fully as 

 primitive as that genus in some respects and greatly modified in 

 others. In Merrianiia the digits are somewhat curved, but in 

 Toretocnemus they seem, judging from somewhat fragmental 



