914 



THE ANIMAL CARRIERS OF DISEASES 



1. Fore feet modified for digging, their claws very large; eyes 



and ears very small; form stout and short; large external 

 cheek pouches — Geomyidce. 



2. Fore feet not modified for digging; their claws normal; eyes 



and ears generally large; form slender. 



(a) Grinding teeth at least four in each jaw; tail long- 



haired, generally bushy, not scaled — Sciuridcs. 



(b) Grinding teeth not more than three in each jaw, or if 



four (Perognathus and Zapus) ; the tail closely haired, 

 sometimes scaly. 



(c) With large external cheek pouches — Heteromyidcs. 



(d) Without external cheek pouches :■ — 



(1) Hind feet not greatly elongated, little if any longer 



than front feet ; tail not longer than rest of body 

 • — MufidcB. 



(2) Hind feet greatly elongated, much longer than 



front feet ; tail much longer than rest of body — 

 — ZapodidcB. 



II. Fur thickly sprinkled with sharp, stiff, spine-like quills- — ErethizontidcB. 



B. Upper front teeth four, the second pair minute and placed directly behind 

 the first pair ; hind legs much longer than front legs ; ears very large ; 

 tail very short (suborder Duplicidentata) — FepovidcB. 



Family Sciurid.e Gray, 1821. 



Squirrels and Marmots. 



The genera of the Sciuridae, which concern us, may be recognized by the 

 following table taken from Swenk: — 



A, Sides without a furred membrane (subfamily SciurincB, true squirrels and 



marmots) . 



I. Tail long, much over one-half of length of body; form slender; 

 coloration usually spotted or streaked. 



(a) Cheek pouches absent ; tail bushy, the hairs growing outward ; 



arboreal (squirrels)- — Sciurus. 



(b) Cheek pouches present, large; tail well haired, but not bushy; 



mainly terrestrial. 



1. Nail of thumb well developed; back conspicuously striped 



lengthwise with five dark and two or four white 

 stripes. 



(a) Premolars in upper jaw one on each side ; back with two 



white stripes; rump rufous; tail with hairs shorter 

 than rest of body (eastern chipmunks) — Tamias. 



(b) Premolars in upper jaw two on each side; back with 



four white stripes; rump greyish; tail with hairs as 

 long as rest of body (western chipmunks)- — Eutamias. 



2. Nail of thumb rudimentary; back striped lengthwise with 



seven lines alternating with six rows of spots, or irregularly 

 and indistinctly spotted with whitish, or plain without 

 either distinct spots or streaks (ground squirrels) — Citellus. 

 II. Tail short, less than one-half of length of body; form stout; coloration 

 always plain. 



(a) Skull highly arched, causing the head to appear convex above; 



tail very short, flattened ; thumb-nail well developed, normal ; 



fur short and full (prairie dogs) — Cynomys. 

 [h] Skull nearly straight, causing the head to appear flat above; tail 



short, bushy, not flattened; thumb-nail broad and flat; fur 



long, coarse, and heavy (woodchucks) — Marmotta. 



B. Sides with a densely furred membrane joining front and hind legs (sub- 



family Pteromyinae, flying squirrels) — Sciuropterus. 



