314 



University of California Publications. 



[Geology 



Dimensions of the Type Specimen 



Tarsometatarsals — 



Length over all 



Length to middle of papilla of tibialis anticus 

 Transverse diameter of head 



129.1 mm. 



22.5 



21.5 



Transverse diameter of foot 



24. 



Least transverse diameter of shaft 



10.8 



Sagittal diameter of head 



17.2 



GERANOAeTUS GRINNELLI, n. sp.* 



Type specimen no. 12175, Univ. Calif. Col. Vert. Palae. Tar 

 sometatarsus in perfect preservation. Resembles Geranoaetus 

 melanoleucus in general, but is slightly more robust and shows 

 superior strength by greater production of the hypotarsal ridge 

 and lower position of the papilla of the tibialis anticus. 



Details of comparison are as follows: Anterior view. The proximal 

 depression is not so deep ; the proximal foramina are very close together 

 and almost on the same level; and the outer attachment of the ligamentous 

 bridge is in the form of a rounded papilla instead of an elongate ridge. 

 The head of the bone at the extreme summit is just equal in width to that 

 of G. melanoleucus, but narrows less rapidly as it merges into the Shaft, 

 The papilla of the tibialis anticus is of about the same size, but is placed 

 farther down the shaft, thus giving a ratio of power to weight arm of 

 23.8% as against 18.4% in G. melanoleucus. The distal foramen is larger 

 and the furrow leading to it is deeper and more perfectly defined. The 

 trochleae are practically identical in the two species. 



Posterior aspect — The most striking differences noted from this aspect 

 are the closer approximation of the hypotarsal ridges and the prolonga- 

 tion of the inner ridge to a greater distance down the shaft. Aside from 

 the stouter shaft, no other difference is appreciable from the rear. 



Proximal articular surface — Prom this viewpoint the fossil form pre- 

 sents three points of divergence from the Recent species. The anterior 

 and the external borders are both indented instead of being nearly straight. 

 The external articular facet thus assumes a more rounded outline. The 

 hypotarsal ridges are produced to a greater degree and, although the space 

 included between them is narrower, the inner ridge seems less deflected 

 toward the median line. The cross-section of the hypotarsal groove thus 

 presents a quite different outline. 



* This species is named in honor of Mr. Joseph Grinnell, Curator of the 

 California Museum of Vertebrate Zoology, and one of the foremost students 

 of the distribution of west American birds. 



