12 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
and about to 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 reéntrant 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. 
TasLe I.—Measurements of eighteen skulls of various species of NEOTOMA 
Be ce | Seen ee oa theives 
No.* Locality. = = 5 Stilley I [les ae Remarks. 
@ | @ /wFris Fk) Palace 
Bee a dsl lue tees 
3442 | South Atlantic States...) 2.10 | 0.72 | 1.08 | 0.27 | 0.77 | 1.40 | floridana; perfectly adult. 
3443 |.-..-- GW ecccrc caeasossene 2050110570) et 105) | OL 2a ON ida) haan ee eae do 
2056 | Saint Simon’s, Ga...-----| 2.00 | 0.68 | 1.02 | 0.25 | 0.75 | 1.30 | floridana ; mature. 
22010 see Gh s< Goon cococ ote see TIEN || WE GES fee She 08255) 02705 |(A25s sae. do 
1682 | Southern States....-.-..- 180) cee celle 0.22 | 0.68 | 1.20 | floridana ; youngish. 
2466 | Georgia ..--.. -.-.--..-.- 1.98 | 0.61 | 0.98 | 0.24 | 0.76 | 1.25 | floridana; nearly adult. 
ZAG I aera Gs ason ce cbanoaaese 1.68 | 0.62 | 0.84 | 0.21 | 0.61 | 1.10 | floridana; quite young. 
PGT | SOHO Di gcsods coessonsesco 1,80 | 0.62 | 0.97 | 0.22 | 0.65 | 1.22 | “ mexicana”; mature. 
3597 | Fort Tejon, Cal ..-2--.--- 1955} OFGS | T02O523) 0768123) oase do 
3098 |------ GW) soncce wocoo oases 1,87 | 0.65 | 0.94 | 0.23 | 0.72 | 1.18 |-----. do 
Bee) dececs GO). csdocsobocoda seed 1845) OF625) 05950326) Onze plee meer do 
BOUGN|seeeee Oyeee cis art sec ce eisee Se ceee | neers OND2 HO! 227 FONTON ten ken Peers do 
SoHE | Seeeee On esse ca-csesese 1592: |) 02165: || 0595 0825: 0275)|| 1420)|/222 = do 
3602 |.----- GbisseSase chsEeecser | 1.90 | 0.68 | 0.95 | 0.21 | 0.70 | 1.20 ]..---. do 
2040 | Santa Clara, Cal.-.-..... {251101 Sees 1.12 | 0.25 | 0.77 | 1.40 | fuscipes ; adult. 
1676 | Charco Escondido, Mex.--.|--....|......]..--..|.-----|-----. 1.20 | ‘‘micropus”; adult. 
6383 | Washington Territory....| 2.10 |...... 1,07 | 0.20 | 0.86 | 1.42 |. “occidentalis”; adult. 
4262 | Deer Creek, ‘‘Neb”....... I 23055 |\ee sees 1.05 | 0.25 | 0.82 | 1.45 | 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. 
tTaken by laying the skull on a flat surface, and measuring perpendicularly from the surface to 
the highest point of the parietals. The same mode of measurement in subsequent tables. 
{ From the tip of the incisors to the back of the condyle. The same, in following tables. 
N. B.—Measurements, throughout this article, are in decimals of the English inch. 
