5D4 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
view. of the fact that out of four Sciurine families only one, Sciwrid@, possesses 
these processes. 
I might extend this comparison of Haplodon with Sciuromorpha into 
many other points of structure, and enlarge upon various Marmot-like char- 
acters; but the indications afforded by the skull seem to me so unmistaka- 
ble, that such course becomes unnecessary. I will only refer, further, to the 
general mode of life, in which, as is well known, the Sewellel agrees with the 
Marmot-squirrel section of Sciuride. It lives in communities in subterra- 
nean burrows, exactly like the species of Cynomys and of Spermophilus. 
Various resemblances, in fact, have caused some authors to refer the species 
to the genus Arctomys, and Rafinesque made it congeneric with Cynomys. 
The condition of the molar teeth, whether rootless and prismatic, or rooted 
and tubercular, usually gives good indications among Rodents. In this im- 
portant respect, Haplodon agrees with Castor ; and although the dental for- 
mule are not identical, yet the only discrepancy is in the presence of a very 
small upper anterior premolar in Haplodon,—a tooth functionally insignificant, 
and, in fact, appearing as if it were far on the way toward suppression. Iv 
comparing Haplodon with Castor, we indeed find many discrepancies, even to 
the presence in the latter of an additional rib and dorsal vertebra, common 
outlet of genito-urinary and digestive systems, peculiarly developed perinzal 
glands, &c ; but we should not lose sight of the fact that the secondary or 
adaptive modifications of Castor, in fitting it for an aquatic life, are impressed 
with unusual strength, and are consequently liable to obscure those points of 
structure more valuable in indicating affinities. Even in some of its soft parts, 
however, Haplodon shows special relationships with Castor, such as the pres- 
ence of a glandular collar formed by the enormous salivary organs. 
I am inclined to consider the Haplodont type as probably the oldest 
among existing Sciuromorphs, and as nearer the ancestral type from which 
also diverged Castor, Anomaluras, and Sciurus with its numerous allies,—the 
latter being the most specialized and diversified forms which have been 
evolved from a primitive stock, and having become its largely predominant 
representatives in the present epoch. This view is strengthened by the 
remarkably restricted geographical distribution of the single known species, 
and the numerical proportion of its individuals,—altogether inconsiderable in 
comparison with the abundance of the typical Scitwride. The simple molars 
of Haplodon, as the very name indicates, may be regarded as further evi- 
