MOLLUSCA. G7 



are either internal intlairiination, eruption, or resembling a catarrh <>r 

 asthma, sometimes terminating fatally. 



The genus Pinna has a somewhat triangular shell, pointed anteriorly, and 

 it includes several s[)ecies which attain to a great size, as P. rudis 

 {pi. 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 manufac- 

 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 cuttle- 

 fish, it is said, " rushes upon her like a lion, and would always devonr 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 out 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 does not regard their nibbles until the piiinoteres or jpinnophylax 

 gives it a bite, which causes it to close the shell, and thus to kill the fisli, 

 some of which is given to the crab as a reward for its watchfulness. Even 

 in the present day similar fables are still recorded by careless com^Dilers. 



Fam. 2. Arcidce. The genus At'ca {pi. 76, ßgs. 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, whicii 

 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 tlie shell is more nearly circular in 

 outline, and the line of the teeth partakes of the curvature of the shell. In 

 Xucula 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 tliirty described 

 species of Area, and about the same number of Pectunculus. The genus 

 Trigonia is by some authors made the representative of a distinct family, 

 which is objected to by Deshayes and Agassiz. A single living species. T. 

 ])ectinata, inhabits the seas of Australia. The fossil species are numerous, 

 and extend from the Lias to the Cretaceous group. A single tertiaiy 

 species occurs in Bolivia. The genus has been illustrated in a masterly 

 manner by Agassiz in his Etudes critiques sur les Mollusques fossiles : 

 Xeuchatel, 1810. Trigonia tJwraclca (Morton's Synopsis, &c. p. 65. 

 pi. 15, fig. 13) is from the cretaceous formation of the United States. 



Fam. 3. Unionida;. To this family the name of Nayades 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 



