44 The Extinct Rodentia of North America. [ January, 
the Rabbits of the Lagomorpha, and I have noted! how that they 
have superadded to the ordinary rodent structure certain points 
which also characterize, the most specialized. Perissodactyla and 
a 
A 
pati 
4 
l 
i 
E 
4 
Artiodactyla among ungulates. The fusion of the inferior part of : 
the fibula with the tibia (found also in the Myomorpha) belongs t to 
the higher types of these orders. The strong intertrochlear ridge 
of the humerus is an especial feature of the groups mentioned, dis- 
tinguishing them from the lower types in all the orders. The 
articulation of the fibula with the calcaneum, mentioned by Mr, 
Alston, is a character of the Artiodactyla. Associated with these i is 
the elongation of the bones of the limbs, especially, the posterior 
one. The modification of the tarsus. in Dipus (the jerboas) evi- 
dently has a direct relation’ to the projectile force transmitted 
through the hind legs in rapid progression by leaping. Here the 
metatarsals are coossified into a cannon bone, though, as there are 
three bones, involved, the result is somewhat different from the 
cannon bone of the Ruminantia. 
| es ee ohn «) Loup 
Wasateli Bridger | Ridel Day. | 1 Fork., 
pete 
| 
| 
IAE ne E (porcapinesj. | 
Hystric | 
Hystrix L. eevee h arhe gestr ss eee | iaaa rails $ I 
SCIUROMORPHA (squirrels). | 
Mylagaulide i | 
Mylagaulus Cope, wile deala of ge eee mebinidele sees >| 2 
|| Fam. ? a 
Heliscomys Cope. ....+.. vhs begun wees b onl arora siepi) T eth se Kee eee 
_ Castorida: i wri i | z | 
Eucastor Leidy - s... - ee ee ee ee ee ls wee eee ee ecee I 
Gaston Joa sad ss paves e hiera pees I 2 I 
Lehyramyide.: | | | 
Plesiarctomys Brav,, 3 ee ee eee 
Syllophodus Co ladak 2 agen Gee ljeeee 
There r | | ane keere 
iride ef $ if d | i 
Meniscomys Co i RE GN ane oe 4 joes 
Gymppiyohos Gs. ; l ke aaa bemp egei 
Sciu "i eeeeee b$ dd | I | 2 i I 
p ESTA ( rats): | | | | 
r MNE oh alan Ho Li TRE A E | Fae 
Eumys Leidy. asters ml Hoy gi enj, I h cheer Page 
Hesperumys Waterh. ju. evalin.. pi. asolteient algei i idw 
Paciculus CARE: hikes» sia ees. ea La, base ba 6,01 Teda 
" Geomyidee. Mann | Dect kere 
Pleurolicus Cope......:. B JQ. Dita. bed ba) oo Shules eit 
Entoppychys Go PE eve ecereccee reese ee ges ranees te eeee enn | 5 sess ee 
LAGOMORPHA HE i | R | 4 
i | el oe p 
ji j iodi sot biagan 
Paleolagas Leidy. bei E P | ents ban caps | 4 | Io fe 
Panolax Co ope... Eere ps | Gus! ja tek i” 
Bes Bane. gone oss wee A ernie, eee ee dJa. sie 
1 Bulletin U. S. Geological Survey Terrs. tv, 362, 1881. 
eae team en 
