ADAPTIVE MODIFICATIONS OF LIMB SKELETON 461 



that the earHest forms known, Mixosaurtis from the Middle 

 Triassic of Lombardy and Shastasauriis, Merrianiia and Toretoc- 

 nemus, from the Upper Triassic of CaHfornia and Nevada, offer 

 very conflicting evidence. One would naturally expect the limbs 

 of these early aquatic forms to show to some extent the characters 

 of their terrestrial ancestors — at least to approach this character 

 more nearly than later forms — and this, indeed, Mixosaiinis does, 

 as Baur has shown (Ueber die Abstammung der amnioten Wirbel- 

 thiere). In this genus the hind limb is as well developed as the 

 anterior and both bear very evident marks of terrestrial origin. 

 With the other Triassic genera mentioned, however, the case is 

 quite different. In all three the limbs are very much specialized 

 and much reduced, but in Toretocnemiis the hind limb is equal 

 to or greater than the fore limb, according to Merriam, while 

 in Merrianiia the hind limb appears quite diminutive in com- 

 parison with the fore limb. When our knowledge of these 

 genera is more complete we shall doubtless find them to be an 

 early offshoot of primitive Ichthyosaurs, in which the limbs 

 have become precociously adapted to aquatic life. The tail of 

 Delphinosaiirus (Shastasaurus) perrini, according to Merriam 

 must have borne a very well developed tail fin, as it shows the 

 sharp deflexion and elongated spines found in later Ichthyo- 

 saurs. The Ichthyosauridae, of the Jura-Cretaceous, have the 

 anterior limb much better developed than the posterior one, and 

 sometimes the latter is very weak. In these forms the tail fin 

 was well developed for locomotion. (See Fraas's figure of /. 

 quadriscissiis , Quenst.) 



The Plesiosaurs are comparatively long-legged and short- 

 tailed and the limbs were undoubtedly used for propulsion to a 

 greater extent than in those forms with a longer tail. While 

 the limbs are evidently adapted for swimming they never 

 become so reduced as in forms which come to use the paddles 

 mostly for purposes of equilibration. Williston has well ex- 

 pressed the condition in this group, " Among the Plesiosaurs 

 the hind limbs are always powerful and well developed, though 

 in all forms known to me they are less powerful than the front 

 ones. Here the relative shortness of the tail has imposed a 

 distinct propelling or sculling function upon the limbs." 



