288 ANNAL8 NEW YORK ACADEMY OF SCIENCES 

 GRAVIPORTAL AND CURSORIAL TYPES OF TIBIA 



Some of the tibio-femoral ratios /T\ given on Plate XXXIV may here 



be grouped as follows: \F/ 



Graviportal Mediportal 



Uintatheriurn 53 Pateosyops 77 



Coryphodon 61 Rhinoceros indicas 79 



Pyrotheriuni 56 Tapirus 80 



Mastodon 69 Pantolambda 76 



Elephas indicus 60 



Brontotherium 54 



Metamynndon 58 



Teleoceras 57 



Subcursorial or Primitive 



Cursorial 



Eohippus • . 1.00 



Mesohippus 1.08 



Phenacodus primaevus 84 Neohipparion 1.17 



" wortinani ... .97 Equus caballus 92 



Euprotogonia 1.01 " scotti 88 



Meni^cotherium .91 Tragulus 1.09 



Procavia 97 Odocoileus 1.16 



Hyrachyns 95 Gazella 1.25 



Sus scrofa 86 Antilope 1 .21 



Eotylopus 96 Antilocapra 1 23 



These figures express the fact noted by Osborn, that, in general, gravi- 

 portal forms have relatively a short tibia and long femur, while cursorial 

 forms have a long tibia and short femur. It is highly probable that in the 

 remote ancestors of all the Placental orders, the tibia was long, perhaps 

 about as long as the femur. The primitive combination of long tibia and 

 short feet is retained in Meniscotherium, Hyrax and to a less degree in 

 Euprotogonia, Phenacodus ivortmani and Eohippus. The steps through 

 which the shortening of the tibia in graviportal animals has been 

 attained may in some cases be discerned. Thus in the Amblypoda, the 

 small and primitive Pantolambda has a tibio-femoral ratio of .76; in 

 Coryphodon, this ratio falls to .61 and in the huge Uintatheriurn to .53 ; 

 similarly in the titanotheres, the ratio drops from .77 in Palceosyops to 

 .54 in the massive Brontotherium. In the Rhinoceroses, it falls from 

 .95 (Hyrachyus) to .79 (Rhinoceros) and .58 (Metamynodon) . 



In the extremely cursorial lines, the long tibia of primitive mammals 

 was usually lengthened slightly, but in the heavier members of cursorial 

 phyla (e. g., Equus, Bison), we observe occasionally a falling off in the 

 ratio. 



The difference between the shortest graviportal tibia ( Uintatheriurn, 

 ratio .53) and the longest cursorial tibia (Gazella, 1.25) is much less 

 extreme than in the metatarso-fe moral ratios, which range from .11 to 



