r40 



TITAXOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



(1) Certain features of original, or syngenetic 

 resemblance survive; (2) the adaptation of structure 

 is rarely exactly analogous, because the functions or 

 movements of the limbs in two unrelated forms are 

 rarety, if ever, exactly analogous. 



For example, the mediportal titanothere Mesnti- 

 rhinus may imitate the mediportal Tapinis; the result- 

 ing limb proportions and forms are closely but not 

 exactly similar. Likewise the femur of the gravi- 

 portal Brontotherium is not exactly similar to that of 

 any other known graviportal form. The differences 

 are due to differences in the gait, step, or mode of 

 progression, as well as to the syngenetic influence of 

 the ancestral forms from which these types respec- 

 tively converged. 



FORMS OF SCAPULA 



That the proportion and form analogies, if not 

 exact, are very close, is demonstrated over and over 

 again in almost any bone we may select. For example, 

 in the scapula there are three chief forms, which are 

 produced repeatedly in difl^erent families — namely, 

 the "mediportal," the "graviportal," and the "cur- 

 sorial."' 



The primitive scapula of Phenacodiis is a suboval 

 type from which that of Tapirus, Mesatirhinus , and 

 other primitive mediportal scapulae are readily de- 

 rived. A feature of the primitive scapula is the 

 prominent acromial process of the spine, which is re- 

 tained in the amblypod Uintatherium but lost in most 



ABC D 



Figure 671. — Five types of scapula 



A, Cursorial (.Mcsohippus); B, primitive (.Phenacodus); C, mediportal (.Tapirus); D, heavy mediportal {Palaeoiherium); all one-sixth 

 natural size. E, Graviportal (Uintatherium); one-eighth natural size. 



The general contrast in the form and proportions of 

 the limb segments between cursorial and graviportal 

 types may be summarized as follows: 



Contrast between cursorial and graviportal types 



Perissodactyla, in which we observe a prominent 

 tuber spinae. 



The mediportal scapula, as illustrated in Tapirus, 

 with graviportal variations seen in Palaeotherium mag- 

 num and in Rhinoceros unicornis, is ovate, elongate, with 

 rounded superior border and with equally developed 

 fossae for the supraspinatus and infraspinatus muscles. 



The graviportal scapula as illustrated in Metamy- 

 nodon, Teleoceras, and Brontotherium is, on the contrary, 

 trihedral, wdth a broad angulate extension of the pos- 

 terior border for the infraspinatus muscles. Its 

 analogies are with the scapulae of Elephas, Uintathe- 

 rium, and other large graviportal animals. 



The cui'sorial scapxila as illustrated in Neohipparion, 

 Antilocapra, Hyracodon, of speed type, is long-necked, 

 fan-shaped — that is, expanded above — with progres- 

 sive enlargement of the infraspinous fossa and corre- 

 sponding diminution of the supraspinous fossa; the 

 superior border is truncate rather than rounded or 

 angulate. 



