CHAMA, CARDIUM AND VENUS 75 
Alaska to southern California. In color it is white 
or pale tinted. 
Psephidia ovalis, Dall, the Oval Pebble-shell, is 
larger, more oval, and more compressed. ‘The 
shell is white and polished. Its range is about 
the same as that of the last species. Viviparous. 
Gémma gémma, ‘Totten, the Gem-shell, is a very 
small bivalve recently introduced into San F'ran- 
cisco Bay, with seed oysters from Chesapeake Bay. 
The shells are nearly round, and are marked with 
delicate concentric ridges. The color is white, or 
they may be tinged with purple. Many specimens 
are not larger than big pin-heads. The inflated, 
trigonal variety is known as var. purpurea, H. C. 
Lea. 
We have now left the great Venus Family, with 
its numerous, thick-shelled representatives, and are 
passing on to those mollusks which resemble the 
more delicate Tellens. 
Our first shell is named Petricola carditoides, 
Conr., the Rock-dweller. Normally the shell is 
oval, with radiating ribs, but it has a habit of bor- 
ing into soft rock, or getting into a hole that was 
there before, and then growing to fit the premises. 
For this reason it happens that specimens differ 
much in external appearance. Sometimes one is 
long and narrow, while its neighbor is shaped lke 
a fat bean. The ligament is external, the hinge- 
teeth strong, though sometimes they are nearly 
obsolete, while the shell becomes thick and rough. 
Color, dingy white; length, an inch or sometimes 
two inches. 
