Troxell — Vertebrate Fossils of Hock Creel; Texas. 615 



From the Equns scotti quarry of J. W. Gidley additional speci- 

 mens of the horse were found, one of which is now mounted 

 in Peabody Museum, making in all about eight skeletons taken 

 from this one hill during a period of thirteen years. The 

 Mylodon-camel quarry yielded all the other material listed, but 

 the skeletons were never so perfectly preserved as those of 

 Equas scotti. 



The use of ratios. — The results of this paper are derived 

 mostly from the use of " ratios." Apparently nowhere in the lit- 

 erature has such an application been made of " ratios," comparing 

 one type to another. Osborn in his "Craniometry of the Equidse," 

 and other writers, have used ratios of one dimension to an- 

 other, on the same skull, calling such a "ratio" an "index." 

 The "index of slenderness" or of "elongation" had already 

 been devised to meet the needs in describing the long slim 

 cannon-bones of Equus calobatus, n. sp. (fig. 4), when in con- 

 versation Professor Osborn suggested the value of the " speed 

 index." He also said that the ratios between homologous parts 

 were useful in showing specific differences, and it is probable 

 that this principle has been applied in his study of the Tita 

 notheres soon to be published . 



Professor Lull has frequently suggested the great constancy 

 of the relation between the teeth and limb bones. This idea 

 was utilized, not by a direct comparison of the teeth to the 

 skeleton, thus getting an index, but by comparing the teeth 

 and skeleton of the fossil to be studied to the teetli and skel- 

 eton, respectively, of a known recent animal, thus getting a 

 ratio. This not only serves to separate out the individuals, 

 but also indicates specific and generic characters. 



The use of " ratios " and " indices " will be more fully illus- 

 trated in the description of the various forms. 



I take pleasure in expressing my gratitude to Professors 

 Schuchert and Lull for their generous assistance in the prepa- 

 ration of this paper. Professor Schuchert made it possible for 

 me to go with the Yale Expedition in 1912, and has done me 

 many other favors. To Professor Lull, through whose inspir- 

 ing personality and interesting courses I was led to take up the 

 study of vertebrate paleontology, I am indebted for criticisms 

 which were of great aid in my work on the material treated in 

 this paper. 



List of Species. 



Class Mammalia. 



Equidae. 



I. Equus scotti (separate quarry) 



o 



E. (Asinns) calobatus, n. sp. 



