MOLLUSCA. 67 
are either internal inflammation, eruption, or resembling a catarrh or 
asthma, sometimes terminating fatally. 
The genus Pinna has a somewhat triangular shell, pointed anteriorly, and 
it includes several species which attain to a great size, as R. rudis 
(pl. 76, jig. 19), which is a foot and a half long. The byssus of several 
species is fine and silky, and six inches or more in length. It is manutac- 
tured at Palermo and parts of Italy into gloves, stockings, and other small 
articles of dress, which are expensive, and kept as curiosities. The mollusc 
is eaten, and it produces small amber colored pearls. 
From a remote period various fables have been current about the pinna 
and certain small crabs which are found in the shell, as in mussels, 
oysters, and other shell-fish. When the pinna opens its shell, the cnttle- 
fish, it is said, “rushes upon her like a lion, and would always devour her 
but for another animal whom she protects in her shell, and from whom in 
return she receives very important services.” When the crab goes ont and 
sees the cuttle-fish approaching, it “returns with the utmost speed and 
anxiety” to the pinna, “ who, being thus warned of the danger, shuts her 
doors and keeps out the enemy.” Pliny’s story has been often repeated, 
according to which small fishes enter the shell to feast upon the animal, 
which meee not regard their nibbles until the pinnoteres or pinnophylac 
gives it a bite, which causes it to close the shell, and thus to kill the fish, 
some of which is given to the crab as a reward “ its watchfulness. Even 
in the present day similar fables are still recorded by careless compilers. 
Fam. 2. Arcide. The genus Arca (pl. 76, jigs. 31, 32) has a byssus in 
some species; the foot is split, the mantle is open, and the anomaly of two 
hearts is presented. The hinge margin has a row of numerous teeth, which 
fit between those of the opposite valve. The shell is rather thick, elongated, 
oblique, and has the beaks distant. Some species, as A. tortuosa, are 
curiously curved. In Pectunculus the shell is more nearly circular in 
outline, and the line of the teeth partakes of the curvature of the shell. In 
Nucula the lines of the teeth before and behind the beaks form an angle 
with each other. 
The tertiary beds of the United States contain about thirty described 
species of Arca, and about the same number of Pectunculus. The genus 
Trigonia is by some authors made the representative of a distinct sail, 
which is objected to by Deshayes and Agassiz. A single living species, iy 
pectinata, inhabits the seas of Austr alia. The fossil species are numerous, 
and extend from the Lias to the Cretaceous group. A single tertiary 
species occurs in Bolivia. The genus has been illustrated in a masterly 
manner by Agassiz in his Etudes critiques sur les Mollusques fossiles : 
Neuchatel, 1840.  Zrigonia thoracica (Morton’s Synopsis, &e. p. 65, 
pl. 15, fig. 18) is from the cretaceous formation of the United States. 
Fam. 3. Unionide. To this family the name of WVayades was given by 
Lamarck, but it is now usually restricted to an order of plants. It includes 
the greater part of the freshwater bivalve shells, which are so abundant in 
the waters of the United States, where they surpass, in number and variety 
of species, those of any other country. They usually live in sand or mud, 
271 
