376 
THE OCEAN WORLD. 
into six and twenty furrows, forming so many corrugated ripples on 
its side. 
Cardium costatum (Fig. 164) is an exotic species which inhabits 
Fig. 164. Cardium costatum (Linnaeus), 
the coast of Guinea and the Senegal, the shell of which, white and 
fragile, is much sought after by collectors, and brings a high price 
when the two valves can be identified as belonging to the same 
individual. 
Along the shores of the Channel and in the Mediterranean there 
are few bivalves more abundant than the several species of the genus 
Donax. They live near the shore in shallow water, burying them- 
selves perpendicularly in the sand. They have the very singular 
habit, considering their apparent helplessness, of being able to leap 
to a certain height and then project themselves ten or twelve inches. 
This may often be witnessed in the case of individuals left by the 
retreating tide ; if seized by the hand, and attempts are made to 
disengage them from the sand, they continue to impress on their shell 
a sudden and energetic movement, aided by the elasticity of their foot, 
which is at once decisive and angular. 
The shell of the Donax is nearly triangular in shape, compressed, 
longer than it is high, regular, equivalve, not equilateral ; the hinge 
with three or four teeth on each valve. 
The animal is slightly compressed, and more or less triangular. H 3 
mantle, which forms two symmetrical lobes enveloping the body, is 
open pretty nearly in all its extent, but is united posteriorly, and 
terminates in two syphons or nearly equal tubes, as in Fig. 128, 
p. 323. One of these tubes serves the purpose of respiration : it is the 
