12 



MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



and aboal (o appear as an island of enamel in the dentine area. This is 

 exactly what occurs in Hesperomys, &c. 



Upon attentive consideration of the subject, we conclude that there is 

 really no essential difference in the molar structure of our several genera; 

 that Neotoma and Sigmodon merely wear off their teeth faster than Hes- 

 peromys does ; that the pattern just described for Neotoma is indicative of early 

 maturity ; that with age the reentrant folds of enamel are ground off from 

 their connection with the general enveloping sheet, and appear as islands of 

 enamel in the general dentine area; and that, finally, these islands are rubbed 

 out, when we have, as a condition of senility, a continuous, depressed dentine 

 surface bounded by a raised, indented wall of enamel. 



We tabulate together a part of our skulls of the several species, both 

 real and nominal. The difference in the length of muzzle of the bushy- 

 tailed species is evident from the figures. 



Table I. — Measurements of eighteen skulls of carious species of Neotoma 



No.' 



344-2 

 3443 

 2056 

 2201 

 1682 

 2466 

 2463 

 2160 

 3597 

 3598 

 3599 

 3600 

 360] 

 3002 

 2040 

 1676 

 G383 

 4262 



Locality. 



South Atlantic States. 



do 



Saint Simon's, Ga 



do 



Southern States. 



Georgia 



do '... 



Sonora 



Fort Tejon, Cal . 



do 



do 



do 



do 



do 



Santa Clara, Cal 



Charco Escondido, Mex. 

 Washington Territory.. 

 Deer Creek, "Neh" 



2.10 

 2.05 

 2.00 

 1.90 

 1.80 

 1.98 

 1.68 

 1.80 

 1.95 

 1.87 

 1.84 



1.92 

 1.90 

 2.10 



2.10 

 2.05 



SB 



0.72 

 0.70 

 0.68 

 0.65 



0.61 

 0.62 

 0.62 

 0.68 

 0.65 

 0.62 



0.65 

 0.68 



cS .3 



S 5 



SB p 

 N 



1.08 

 1.05 

 1.02 



0.98 

 0.84 

 0.97 

 1.02 

 0.94 

 0.95 

 0.92 

 0.95 

 0. 95 

 1.12 



1.07 

 1.05 



"2 -2 



a 



0.27 

 0.25 

 0.25 

 0.25 

 0.22 

 0.24 

 0.21 

 0.22 

 0.23 

 0.23 

 0.26 

 0.22 

 0.25 

 0.21 

 0.25 



so — 



5 S 



0.77 

 0.77 

 0. 75 

 0.70 

 0.68 

 0.76 

 0.61 

 0.65 

 0. 76 

 0.72 

 0.71 

 0.70 

 0.75 

 0.70 

 0.77 



0.20 

 0.25 



0.86 

 0.82 



SB 3 



1. 40 



1.30 

 1.25 

 1.20 

 1. 25 

 1.10 

 1.22 

 1.23 

 1.18 

 1.22 

 1.12 

 1.20 

 1.20 

 1. 40 

 1.20 

 1.42 

 1.45 



Remarks. 



floridana; perfectly adult. 



do. 



floridana ; mature. 



do. 



floridana ; youngish. 

 floridana; nearly adult. 

 floridana; quite young, 

 "mexicana"; mature. 



do. 



do. 



do. 



do. 



do. 



do. 



fuecipes ; adult. 

 " micropus"; adult, 

 "occidontalis"; adult. 

 cinerea; adult. 



*Of the specimen, in the register of the National Museum, Smithsonian Institution, and on its 

 label. The same with regard to the numbers in the first column of each table throughout the present; 

 monograph. 



t Taken by laying the skull on a flat surface, and measuring perpendicularly from the surface to 

 tho highest, point of the parietals. The same mode of measurement in subsequent tables. 



t From the tip of the incisors to the back of the condyle. The same, in following tables. 



N. 1!. — Measurements, throughout this article, are in decimals of the English inch. 



