238 U. S. P. R. R. EXP. AND SURVEYS ZOOLOGY GENERAL REPORT. 



the species. It is frequently necessary, however, to take into consideration the age of the 

 animal, as, in consequence of the different elements of the teeth not being parallel to each 

 other, they vary in their respective distances with different degrees of attrition. Again : if the 

 enamel coats the exterior of the crown, and on the grinding surface is studded with tubercles, 

 when these are ground off, we may find an external frame of enamel enclosing a space of dentine, 

 which, if the enamel has dipped down deep into its substance, may still retain islands of the 

 latter component. This condition has given rise to much confusion, and added numerous 

 synonyms to science. 



Most Rodents have the tibia and fibula distinct. In the Mwridae and Leporidae, however, 

 they are anchylosed below. All have clavicles, with a few exceptions among the Hystricidae, 

 although in the hares they are very small. The normal number of digits is five to each foot ; 

 the thumb, however, is generally more or less rudimentary ; on the hind foot the whole five are 

 present in most genera, though in some only four are evident-; and in others, as Dipus, Cavia, 

 and Dasyprocta, there are but three. All Kodents, but the cavies and hares, can use the fore 

 feet as instruments of prehension in conveying food to the mouth. 



The intestinal canal is generally long, and provided with a distinct coecum, except in the 

 Myoxina, which is without it. None of the North American Rodents, as far as known, are 

 without a coecum. 



The systematic arrangement of the Rodentia, with its numerous pseudomorphous forms, has 

 been a subject of much discussion among zoologists, whose views have varied materially, 

 according as different characters were selected as the basis of classification. Mr. G. R. Water- 

 house, 1 however, has been most successful in furnishing a natural system by which the zoological 

 peculiarities of different groups tally remarkably well with the chief points in their geographical 

 distribution. The structure of the skull, and especially of the lower jaw, furnished him with 

 characters readily appreciable. In Charles worth's Magazine, he arranged the Rodentia in three 

 sections Murina, Nystricina, and Leporina; in Johnston's Physical Atlas, however, the more 

 natural series is adopted of Sciuridae, Muridae, Hystricidae, and Leporidae. Geomys, previously 

 placed by him among the Arvicoline section of the Muridae, he sets aside with Perognathus, 

 Dipodomys, Saccomys, and Heteromys, as forming a natural group of uncertain position, and in 

 the Natural History of Mammalia, gives them the provisional name of Saccomyina. In this 

 I agree fully with him, the group having certain characters common to all, and exceptional to 

 nearly all others I would, however, raise it to the rank of a family, and as such, of equal 

 prominence with the four first mentioned above. 



The characters of these groups are to be found in the general form, presence, or absence of 

 cheek pouches ; form of the tail ; shape of teeth and skull, especially of the lower jaw, &c. 

 They may be given briefly as follows : 



I. LEPORIDAE. Incisors ^, molars 6 ~ | or ^, rootless. Skull with the two optic foramina 

 united. The lower jaw with very flat rami, which are of great size ; Symphysis menti nearly 

 horizontal ; condyloid process very high and broad ; the coronoid represented by a mere ridge 



1 Observations on the Rodentia, with a view to an arrangement of the group founded upon the structure of the crania. By 

 G. R. Waterhouse. Charlesworth's Mag. of Nat. Hist. Ill, 1839, 90, 184, 274, 593. Also, in Annals and Magazine of 

 Nat. Hist. 



A Natural History of the Mammalia. By G. R. Waterhouse. Vol. II, containing the order Rodentia. London, 1848. 

 H. Bailliere. 



Table of the orders Rodentia and Ruminantia, Philology and Zoology, No. 5. In Keith Johnston's edition of Borhaus' 

 Physical Atlas. Folio. Edinburgh, 1849. 



