554 



MONOGBAPnS OF NORTH AMERICAN EODENTIA. 



view of the fact that out of four Sciurine families only one, Sciurida, possesses 

 these processes. , , ,, 



I Tiight extend tliis comparison of Haplodon with Sciuromorpha into 

 many other points of structure, and enlarge upon various Marmot-like char- 

 ncterc ; 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 Sciurida. It lives in communities in subterra- 

 nean burrows, exactly like the species of Cynomys and of Spernwphilus. 

 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- 

 mulae are not identical, yet the only discrepancy is in the presence of a very 

 small upper anterior premolar in Haplodon, — a tooth functionally insigniGcant, 

 and, in fact, appearing as if it were far on the way toward suppression, lo 

 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 perinaeal 

 glands, &c ; but we should not lose sight of the fact that the secondary or 

 adaptive modificativ..is 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, mch 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, Anomalurus, 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 Sciuridte. Tiie simple molars 

 of Haplodon, as the very djimc indicates, may be regarded as further evi- 



