452 MARINE INVERTEBRATES 
Genus Saxidomus 
S. nuttallii. The last of the larger west-coast Veneride which we 
shall examine. The genus is very closely allied to Tapes, and no doubt 
might better be considered as its subgenus. The shell is brownish- 
white, three to four inches in length, and heavy, rough, and coarse-look- 
ing, with irregular concentric ridges. There is no lunule, but a large 
external ligament is very prominent just back of the beaks. Within it 
is white, with a deep pallial sinus. Just below the ligament area is a 
zone of translucent, agate-like shell-structure. This large clam, with 
several others of the same genus, lives in shallow water near shore. 
FAMILY PETRICOLIDE 
Genus Petricola 
P. pholadiformis. Along the Jersey coast, especially about Atlantic 
City, there are small patches of clay or hard tenacious mud which lie 
just at the edge of the sea. As the beating of the surf gradually 
encroaches upon these hard fragments of once extensive clay-beds, many 
specimens of this curious bivalve mollusk are washed 
out from their burrows in this clay and cast upon the 
sandy beach. The animal has long siphons which are 
united about one third of the way to their ends. Al- 
though this creature is allied to the Veneride, the degree 
nh of mantle fusion present is much greater, leaving in this 
Win case only a fairly long slit through which the foot may 
i protrude. The shell is very dissimilar to that of any of 
the Veneride, and in many respects, judging from a 
conventional standard, is a remarkable form. It is ex- 
ceedingly thin, is of a chalky texture, gapes widely pos- 
teriorly, and is ornamented with transverse ribs, which 
are feeble upon the long produced posterior portion of 
the shell, but strong in the anterior part, where they are 
crossed by the coarse lines of growth, which appear like 
vaulted scales upon the ribs. The color is a dull white. 
There are two cardinals in each valve and no laterals. 
Petricotaproua. Length one and a quarter to two and a half inches; 
formis. height one half to three quarters of an inch. This 
; ; species passes its entire life in the burrow it has made 
for itself in the clay. Its only motions are made in climbing to the en- 
trance of its burrow or in retreating far out of sight within its depths. 
Its siphons aretipped with a dark or drab color asa measure of protection 
from the predaceous crabs which range about the mud surfaces near 
tidal marks, and are always upon the lookout for succulent bits of flesh. 
P. carditoides. A petricola of the Californian coast, very similar in its 
habits to the east-coast form. It bores, however, not only into clay, but 
also into soft rocks in order to effect a permanent lodgment. Often this 
species occupies a hole which it has discovered already existing ; in this 
case its shell grows to fit the surroundings. The hinge-teeth and sculp- 
turing are frequently reduced and sometimes are quite obsolete; but 
the shell has the chalky, thin texture that is characteristic of the genus. 
