MOLLUSCA. 63 



widely distributed in a recent and fossil state, and is not confined to any- 

 particular country or climate. 



These shells are free, and live upon a surface of mud or sand, from a 

 trifling- depth to twenty fathoms, and they are fished up as an article of food. 

 Tliey present several distinct forms, and many of them possess great beauty. 

 Pecten pleuronectes is named after the generic name of the flounder, 

 because one side is dark colored, and the other white. P. jacobteus was 

 formerly worn by pilgrims who had visited the Holy Land. P. cpiinque- 

 costatus occurs fossil in the cretaceous deposits of Europe and America, 

 and there are numerous species belonging to the tertiary formations of the 

 United States. Lima, a genus of free shells, is allied to Pecten. 



Spondylus {pi. 76, Jig. 35) is a genus of attached, rough or spiny, and 

 usually heavy and finely colored shells, allied to Pecten and Ostrea. There 

 are two strong teeth in each valve, and a depression for the ligament. 

 Tlinnites is allied to Pecten and Spondylus, and possesses the peculiarity of 

 being free until it attains a certain size, when it becomes permanently affixed. 



Fani: 5. Avicidldce. This family includes the shells from which most of 

 the pearls of commerce are obtained. They are allied to the two preceding 

 families, and most of the genera are byssiferous, with pearly shells. The large 

 well known shell (j;Z. 76, jig. 20), which sometimes attains a size of ten 

 inches, produces the finest oriental pearls, as well as most of the mother-of- 

 pearl which is used in the arts. It forms the genus Meleagrina, Lamarck, 

 although it is now considered not to be distinct from Avicula, and it is there- 

 fore named Avicula mar garntifera.^ Linn. It inhabits the Indian seas. Avicula, 

 according to the celebrated anatomist Poli, has the mantle unclosed, and 

 fringed with tentacular appendages. The foot is small and secretes abyssus. 



The genus Malleus {31. malleus., Linn.,_/j»Z. 76, fig. 26) is remarkable for 

 having the hinge mai'gin extended in some species in the antero-posterior 

 direction. The shell is very irregular, the foot secretes a byssus, and the 

 mantle has a fringe of small tentacles. Perna {fig. 30) has an irregular 

 shell, hinge straight, with a row of transverse furrows for the insertion of 

 the ligament. The byssus passes through a gaping vacancy in the front 

 of the shell. The genera Malleus, Perna, Vulsella, Crenatula, Catillus 

 Inoceramus, and some others, are placed by some authors in a distinct 

 family, MalleidcB. 



Pearls are secreted upon the inside of the shell, or in folds of the mantle, 

 the latter being the most regular; and as their quality depends upon that 

 of the nacre, those shells which have this of a fine quality produce the best 

 pearls. The pearls of common oysters are rough concretions of no 

 commercial value, and similar concretions are sometimes formed by univalve 

 species, the mantle of which has, of course, the power of secreting the 

 calcareous matter of the shell. Although pearls are formed out of the same 

 material as the shell, a bead turned out of the latter has not their peculiar 

 lustre, because the arrangement of the material is different, the successive 

 layers being plane in the shell and spherical in the pearl. On this account 

 shaping an irregular pearl does not alter its lustre. Irregular pearls are 

 sometimes worn without being shaped, when the form is agreeable. Pear- 



26V 



