404 THE HORSE OF ANAU IN ITS VARIOUS RELATIONS. 
among asses distinguishes this animal from those. The transverse diameter 
of the skull and the form of the nasal bones conditioned thereby are like those 
in the horse, as is also the eye-socket. The perfectly preserved teeth show that 
the length of the upper jaw is 34 per cent of the length of the skull. Riitimeyer 
finds for the horse, elsewhere, 32 to 35.6; for the ass 35 to 38.5 per cent. Thus 
it should be a horse. Also, the relation of the premolar row of the lower jaw 
to the dental row, which in the horse is 51 to 53 per cent and 49 in the ass, is 52 
per cent in the skull from the lake-dwelling, thus again as in the horse. Only 
the occiput, says Riitimeyer, looks like that of an ass. And he closes his obser- 
vations: “‘ Notwithstanding all the uncertainties which seem to attach to these 
measurements, not only on the teeth but on the skull as well, certainly derived 
from nature, there remains in my mind no doubt that the skull from the lake 
belonged to an ass.”’ 
The kindness of Doctor Lehmann, Director of the Swiss Landesmuseum in 
Zurich, enabled me to make a direct comparison of the skull from Auvernier with 
the mummified skull from Abadieh and with the skulls from the Somme which 
I studied in the Museum of Natural History in Paris. 
During this investigation there arose again the question which I had asked 
inyself before, during the study of the craniology of the ruminants: What are 
the really decisive criteria of species, and what the incidental characteristics brought 
into existence by causes acting during individual life? At last I came to the reali- 
zation that a conclusive method of discrimination did not exist; that all those 
in use might be said to be wholly empirical, in part, indeed, dependent on the 
personal perception and feeling of the individual student, and therefore not scien- 
tifically established. Nor have I succeeded—through lack of material, fresh heads 
and numerous skulls of asses—in adding much that is new; but I believe that I 
have thrown some light upon the causality of some of these relations, and have 
tried to incite to a more scientific treatment of the question. 
CRANIOLOGICAL DIFFERENCES BETWEEN THE ASS AND THE HORSE. 
The older authors, as De Blainville in his Ostéographie, and Cuvier, do not 
supply what is really needful for the comparison in question. [L. Rutitimeyer 
has opened the way here, too, as in many other branches of paleontology. His 
‘“Beitrage zur Kenntniss der fossilen Pferde,”’ etc.,* was the first work worthy 
of note on the fossil remains of the genus Equus, but he did not treat of the differ- 
ences between the horse and ass till in his second treatise ‘‘The Horses of the 
Quarternary epoch’’t in the same year as the studies of Frank mentioned above. 
In the meantime this relation had been discussed by R. Owen in his “ Description 
of the Cavern of Bruniquel and its Contents,’’{ in which are beautiful plates 
representing the teeth characteristics of the horse and ass. 

*Riitimeyer, Beitrage zur Kenntniss der fossilen Pferde u. zur vergleichenden Odontographie der 
Huftiere ueberhaupt. Verh. Naturf. Gesellsch., Basel, Bd. 111, 4, 1863. 
{Riitimeyer, Weitere Beitrage z. Beurtheilung d. Pferde d. Quaternar Epoche. Abhandl. d. Schweiz. 
paleontol. Gesellsch., 1, 1875. 
t Owen, Philosoph. Transactions, vol. 159, 1869, pp. 517-557. 
