CONCHIFERA. 731 



25 bis, fig. 7, Cuv. R. Ani., ed. ill., Moll. PI. 85, fig. i; the pearl-shell. 

 The shell is sometimes four-cornered, with round margin, greenish-brown 

 striped with white spots, nacreous within, yellow or blueish white. This 

 conchifer produces the finest pearls, and is found in the Persian gulf and 

 in the Indian ocean at Ceylon and Borneo, in the straits of Malacca, 

 &c. The largest pearls are found in the deepest places, as in the Persian 

 gulf near the island Kharrak, where they lie on very deep banks. The 

 Persian pearls are hard, and consequently more esteemed than those of 

 Ceylon, which are often subject to desquamation. 



B. Dimyaria. Two muscular impressions, mostly remote, 

 sometimes approximate. Ligament of valves external in most. 



Family Y. Mytilacea. Mantle cloven anteriorly. Foot conical 

 or tongue-shaped, byssiferous. Two muscular impressions in each 

 valve remote. Anterior adductor muscle often very small. 



Pinna L. Shell fragile, thin, often imbricato-squamose, elongate, 

 triangular, angustate towards the points. Hinge edentulous ; liga- 

 ment marginal. Animal ( Chimcera POLI) with foot vermicular, 

 subulate, transversely rugose, delivering at its base the byssus 

 of long, dense, silken filaments. Anterior adductor muscle near the 

 apices, posterior larger, subcentral. A conical contractile appendage 

 at the posterior part of mantle (trachea POLI). 



The shells are large, triangular, thin, and very small at the point ; along 

 each shell there runs lengthwise a keel-shaped eminence sometimes feeble, 

 for each is formed of two surfaces that meet at an obtuse angle. The 

 animal penetrates the sand with the point of the shell, and fastens itself 

 above it by the byssus to other objects. The silky byssus-threads of species 

 from the Mediterranean are used in Calabria and Sicily as guards for 

 gloves, stockings, purses, &c. 



Sp. Pinna noMlis L., Pinna muricata POLI, Testacea utr. Sic. Tab. 34, fig. 

 i, BLAINV. Malac. PI. 64, fig. i ; Pinna rotundata L. (and P. squamosa 

 GMELL., LAM.), LISTER Tab. 374, fig. 215, Encycl. m&th., Vers. PL 200, 

 fig. 2, &c. Many fossil species of this genus from the secondary, and some 

 from the tertiary formations are known ; to the last belongs Pinna niargari- 

 tacea LAM., Ann. du Mus. ix. PI. 17, fig. 3, from the calcaire grossier. 



Mytilus L. (in part), BRUG. Shell elongate, not squamose, 

 rather smooth, equivalve, with points placed forward, hinge edentu- 

 lous or with two teeth obsolete. Ligament dorsal, linear, received 

 in a narrow, elongate, marginal sinus. Two muscular impressions, 

 the anterior very small. Animal (CaUitriche POLI) with foot 

 Ungulate, canaliculate, byssiferous, the mantle concrete posteriorly 



