Troxell — Vertebrate Fossils of Rock Creek, Texas. 619 



Equus calobatus, n. sp. — In the quarry which produced the 

 greater variety of specimens, limb bones of Equus (fig. 4) were 

 found which were unusually long and slender. Associated 

 with these were various parts of the skeleton which correspond 

 closely in size. This specimen, so unique in the form of the 

 limbs, and so distinct from E. scotti and E. caballus, may well 

 constitute a new species. Because of the close similarity to the 

 recent ass, fossil specimens of which are almost unknown, this 

 type will be designated Equus (Asimts) calobatus, n. sp., the 

 specific name meaning " he who walks on stilts." 



The limbs of the new type offer a most interesting study, 

 and it is upon these that the species is founded. The cannon- 

 bones in particular are most unusual in the development of 

 their great length. The tibia is not so long, but in general it 

 is more slender than that of E. caballus, although the latter, as 

 represented by the Arabian horse, has a higher speed index. 



The cannon-bone of the fore limb is 19*4 per cent longer 

 than that of the Arab horse, and it is over a half longer than 

 that of Equus asinus. The length of this cannon-bone in 

 E. calobatus, whose diameters give an average ratio of 85*4 per 

 cent of that of E. scotti, is actually greater in length by more 

 than 4 cm . 



Of all the species studied, the speed index is highest in the 

 cannon-bone of the ass (8*68) ; next to it is that of the new 

 species, in which the index is 8*57. It is very high in either, 

 but considering the greater size of the fossil (a half taller, and 

 therefore probably over three and a third times as heavy), its 

 index is exceedingly high. Compared to the Arabian horse, a 

 good cursorial type, the slim fossil shows an index 18 per cent 

 higher, and compared to the ponderous E. scotti, it is 44 per 

 cent higher. 



The speed index is found by dividing the length by the 

 transverse dimensions of the shaft. The shaft measure in 

 E. asinus is particularly small ; this makes the speed index 

 correspondingly high. If the average of all the diameters (see 

 table, page 620) be taken and divided by the length, the ratio 

 may be called the " index of slenderness." This index shows 

 that E. calobatus has the slenderest cannon-bone of all, the 

 order of slenderness being : 



E. calobatus 14*3 per cent 



E. asinus 15*7 " 



E. caballus 17*0 " 



E. scotti 19-0 " 



The reciprocal of the " index of slenderness " may be taken as 

 the " index of elongation." 



