426 MARINE INVERTEBRATES 



brown. Length two to two and a quarter inches ; height two inches. 

 A. pexata is one of the few pelecypods which have red blood, a fact 

 which accounts for its popular name of " bloody clam." 



A. ponderosa. The most prominent Area upon our Atlantic coast, 

 especially south of Virginia. The beaks of this shell are very promi- 

 nent, and do not approach each other very closely on account of the 

 wide dorsal margin of the shells, the large space between the beaks 

 being occupied by an external ligament. The hinge-teeth are smaller 

 in the middle. There are from twenty-five to twenty-eight ribs ; the 

 ventral margin of the valves is somewhat contracted in the middle. 

 Length two and a half inches; height two inches. Very heavy and 

 solid. This exceedingly common species is often cast upon the beaches 

 south of Hatteras in numbers beyond computation. In life it is covered 

 with a heavy, coarse, velvety epidermis, almost jet-black in color. 

 (Plate LXXVIII.) 



A. now. The well-known "Noah's-ark" shell, a common species 

 along the shores of the Southeastern States. It also occurs in the 

 Mediterranean. Unlike most areas, it spins a byssus, by means of which 

 it attaches itself to the under surfaces of stones at low water. The 

 hinge-margin is perfectly straight and regularly toothed. The beaks 

 are high and are situated very far forward. The dorsal margin of the 

 valves is strikingly large, and is marked by lattice-like grooves ; it forms 

 a concave surface over an inch in width between the umbones. The 

 shell is strongly ribbed without. The ventral margin of the valves is 

 sinuous and gapes slightly at a central point for the accommodation of 

 the byssus. (Plate LXXVIII.) 



FAMILY MYTILID.E 

 GENERA Mytilns, Modiola 



The MytiUdte are a large family, including the mussels, so com- 

 monly found between tides upon all shores. The two ordinary 

 genera Mytilus and Modiola, which often occur associated to- 

 gether upon our eastern coast, may readily be distinguished one 

 from the other by the position of their beaks. In the former 

 genus the beak is terminal, that is, it is situated at the very tip- 

 end of the shell ; in the latter genus it is not quite terminal. 

 Aside from this simple feature, the genera are practically the 

 same, the animals being identical. Mytilus and Modiola, com- 

 monly known as mussels, have acquired the stationary habit and 

 are provided with a relatively small foot (in some species amount- 

 ing to no more than a mere tubercle) ; but in place of a useful 

 foot is a well-developed byssogenous gland, which secretes an ex- 



