no. 2086. SOME MAMMALS OF THE PLEISTOCENE— HAY. 565 



The skull of the domestic ass may in the great majority of cases 

 be distinguished from that of all other equids. Grevy's zebra 

 possesses craniometrical characters which set it off quite decidedly 

 from other horses and zebras. The writer believes that measure- 

 ments and the indices derived from them show plainly that two 

 species are mingled in different proportions in our different races 

 of domestic horses. 



2. Not all species can be distinguished by craniometrical methods. 

 It is reasonable to suppose that two or more species might have 

 skulls and skeletons of practically the same size and proportions 

 and yet differ greatly in external characters and perhaps in details 

 of structure of the teeth. 



3. In cases where only single skulls of two or more supposed 

 species are at our disposition, as in the case of most fossil horses, 

 the measurements and indices must be employed with circumspection. 

 The measurements of one skull may differ considerably from those of 

 another and yet both belong to the same species, for they may present 

 the extremes which arise from individual variation. Not too much 

 dependence should be placed in any one measurement or in any 

 single index. All ought to be considered, and all other characters 

 ought to be considered in coming to a conclusion. 



4. It is difficult to say which measurements and which indices 

 are of the most importance. The basilar length, the cranial length, 

 the facial length, the width at the rear of the orbits, and the indices 

 derived from them are of prime importance. In the application of 

 these to special cases sometimes one index may be of special value; 

 in other cases another index may be decisive. 



5. The value of still other indices than those considered in this 

 paper, such as that expressing the angle included between the axis 

 of the cranium and the axis of the face, the index showing the rela- 

 tion of the length of the tooth line to the basilar length, the index 

 expressing the relation of the protocone to the length or width of 

 the tooth, ought to be tested on the Przevalsky horse, the zebras, 

 the chigetai, the kiang, and the domestic ass. 



4. ON SOME RESULTS OBTAINED BY W. SOERGEL. 



W. Soergel has written an interesting paper entitled Die Pferde 

 aus der Schotterterrasse von Steinheim a. d. Murr. 1 This author 

 has reached certain results which he regards as of great value in 

 determining the phylogenetic position of the various horses of the 

 Pleistocene of Europe (p. 743) and the age of the deposits in which 

 horse teeth may occur (p. 746). While Soergel's results are applied 

 by him only to the horses of the Pleistocene of Europe, they might 

 be supposed to be applicable to those of North America likewise. 



iNeues Jatarb. Min., etc., Beilago, vol. 32, 1911, pp. 740-761, pis. 33, 35. 



