CHAMA, CARDIUM AND VENUS 63 
Cardium quadrigenarium, Conr., the Forty. 
ribbed Cockle, is another southern species, and 
grows in deep water to a remarkable size. In spite 
of its name the ribs are usually rather more than 
40 in number. While the shell is young the ribs 
are comparatively smooth; but when older they 
are set with yellow teeth, or short horns, giving 
the shell a very peculiar appearance. The strong 
crenulations on the edges of the valves are also 
yellow, especially in adult specimens. 
Sérripes gronlandicus, Gmel., the Greenland Ser- 
ripes, is an Arctic species coming down to Cape 
Cod and Puget Sound, like C. cihatum. Its hinge- 
teeth are small and almost obsolete, and the surface 
is smooth or only slightly radiately striate. 
Protocardia centifilosa, Cpr., the Hundred-lined 
Cockle, has a small, thin shell, whose surface is 
sculptured with very many exceedingly fine lines. 
Umbones prominent, outline circular, length 28 
mm., range from B. C. to San Diego. A var. 
richardsonu, Whiteaves, reaches from the north 
to the coast of Oregon. It is very small and rare. 
From the interesting Heart-shells we turn with 
reluctance, but our journey will now take us away 
from the ocean for a little season. 
Our first shell will be an extremely small 
one, represented in Figure 49. The name 
of this little creature is Pisidium abditum, 
Hald., the Hidden Pea-shell. It lives in fresh 
water, and has a thin, oval, brown shell, marked 
by minute lines of growth. It is widely distrib- 
uted, being found even in New Mexico, and natur- 
Fig. 49 
