452 



MARINE INVERTEBRATES 



GENUS Saxidomus 



S. nuttattii. The last of the larger west-coast Veneridce 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 PETEICOLIDJE 



GENUS Petricola 



P. pholfidiformis. 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 Venerida, the degree 

 of mantle fusion present is much greater, leaving in this 

 case only a fairly long slit through which the foot may 

 protrude. The shell is very dissimilar to that of any of 

 the VenericUe, 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. 

 Length one and a quarter to two and a half inches ; 

 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 are tipped with a dark or drab color as a 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. 

 JP. carditoides. A petricola of the Calif ornian 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. 



Petricola pholadi- 

 formis. 



