THE SMALLER SEA-SHELLS 211 
appropriate them, and they may appear to be 
running off without any apparent means of pro- 
pulsion. The shell is shaped like a short, stout 
thorn, and varies in length from one-fourth to 
one-half of an inch. The whitish or brownish 
whorls are eight or ten in number, and are marked 
by slight, spiral grooves. 
Bittium quadrifilatum, Cpr., the Four-lined Bit- 
tium, is a southern species, with a smaller shell 
than the last. In shape it is a regular but very 
slender cone, and the whorls are marked by four 
equal spiral threads, which coil over slight 
cross-ribs. 
Seila assimilis, C. B. Ad., the threaded Seila, 
(Cerithiopsis assimilata), has a slender shell, 10 
mm. long, consisting of ten whorls around which 
run three spiral ridges, winding from the apex to 
the aperture. It lives from Monterey southward. 
