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 Arca 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.nocw. 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 LXXVIILI.) 
FAMILY MYTILIDE 
GreneRA Mytilus, Modiola 
The Mytilide 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- 
