362 



GEOLOGICAL BIOLOGY. 



biped reptiles, three-toed and with avian pelvic structure ; 

 flying reptiles, with beaks instead of teeth ; birds with teeth, 

 and birds with long vertebrated tails. 



So many points of combination of features have been seen 

 in the Mesozoic fauna, which are now only found separated 

 in the two great classes Aves and Reptilia, that zoologists 

 have been forced to provide an intermediate group to include 

 these ancient types, or to expand and combine the two classes 

 into the one superclass Sauropsida of Huxley. 



Specialization of Five Fingers in Reptiles and its Relation to 

 Later Specializations. The principle of synthesis, or combina- 

 tion, in an early type, of the characteristics of two or more sepa- 



TV 



'FiG. 120. Left forefoot of A, Phenacodus primavus Cope, Eocene ; /?, Hyracotkerium venti- 

 Colum Cope, Eocene ; C, Paleotherium medium Cuv., Oligocene ; ./?, Anchitherium aureli- 

 anense Blainv , Miocene ; , Hippotherium gracile Kaup., Pliocene ; F, Equus caballus L., 

 Recent. / = lunar ; m = magnum ; p = cuneiform ; o scaphoid ; / = trapezoid ; tz 

 trapezium ; u = unciform ; I-V = ist to 5th finger or metacarpal bones ; me metacarpal. 

 (Steinmann and Doderlein.) 



rate types of a later stage, is seen in the case of the Permian 

 reptile Mesosaurus tumidus Cope, in which five tarsals are 

 present, rather than four the normal number of later rep- 

 tiles. Such a fact shows, according to Cope, that five is the 

 primitive number of tarsals, and that four is a specialization 

 just as we find in general in the evolution of paws, feet, and 

 hands the full number of parts was provided before the spe- 

 cialized reduced number was evolved. The fewer number of 

 fingers or of bones, entering into the mechanism of the foot 

 or hand, is the result of selection and specialization of parts 

 rather than the direct production of any new function or part. 

 The Eocene Phenacodus primcevus Cope illustrates this princi- 



