142 CLASS I. TROPIOPODA. ORDER II. UNIMUSCULOSA. 



entirely coated with a firm nacre, it is composed of a number of perpen- 

 dicular fibres, disposed either in one laminal plate, or in several, one 

 upon the other; and the nacre is only deposited in the central con- 

 cavity of each valve, in the part occupied by the animal. This ex- 

 tended growth of the shell beyond the seat of animal existence is said 

 to be very elastic during life, but when taken out of the water and dried 

 it becomes hard and brittle. The shells of the Aviculacea have all this 

 fibro-laminal structure, particularly those of the genus Avicula. The 

 apical extremity of the valves is sometimes divided by a longitudinal 

 suture, but this only occurs in one or two species. 



The shell of Pinna may be described as being equivalve, oblique, 

 wedge-shaped and longitudinal, having the outer surface either smooth 

 or muricated ; the umbones are acute and terminal ; the anterior side of 

 the shell is a little gaping, but the posterior side often gapes widely. 

 The hinge is lateral and without teeth ; the ligament, which is partly 

 internal, being continued throughout its entire length. The muscular 

 impression is compound, as it exhibits the mark of a small anterior 

 accessory cartilage. The Pinnae have comparatively a large shell ; their 

 byssus is composed of fine glossy silken threads, and in some parts of 

 Italy have been fancifully used in the manufacture of gloves. They are 

 usually found partially buried in the sand, or in the crevices of rocks, 

 with the pointed extremity of their shells downwards. 



Examples. 



pi. cm. 



Pinna serrata, Solander, MSS. Sowerby, Genera of Shells, No. 26. 

 Deshayes, new edit, of Lamarck, vol. vii. p. 67. 



PI. CIV. 



Pinna nigrina, Lamarck, Anim. sans vert., new edit., vol. vii. p. 66. 



Enc. Meth., pi. 199. f. La, b. 

 Pinna nigra, Chemnitz. 



