A V I C U L A. 



Genus AVICULA, Kld,i. 

 Tenia irregidaris, nunc orbiculuris 7umc oblique trigona, 

 latere antico plus minus elongato-alato, fibro-lamina- 

 ris, inajuivahis, lacuna pro bijsso, intus plerumque 

 margaritaced. Gardo edentulus, sed tuberculo obscuro 

 in uiraque valvd infra umbones plerumque instructus. 

 Ligamentum lineare, in medio plus minus dilatatum. 

 Impressio muscularis centralis. 

 Shell irregular, sometimes orbicular, sometimes obli(iMely 

 triangular, with the anterior side more or less elou- 

 gately winged, fibro-laminar, inequivalve, having an 

 opening for the byssus, interior iieuerally pearly. 

 Hinge toothless, but mostly furnished in each valve 

 beneath the umboes with an obscure tubercle. Liga- 

 ment linear, more or less dilated in the middle. Mus- 

 cular impression central. 

 Under this head are included the Lamarckian Avicula 

 represented by the characteristic A. macropfera and /mle- 

 roptera, and Mehagrina, of which the great Pearl Oyster, 

 A. margaritifera, is the type. Though differing materially 

 in shape, the animal is the same, and a close inspection of 

 the shell will show that even the roundest form of Melea- 

 grina possesses the rudiment of a winged process. Like 

 all shells that live attached to rocks by a byssus, the Avi- 

 cula vary greatly in general form, and the same species is 

 often so variable in growth that it is not easy to identify it 

 by description. In substance the Amculcs are fibro-lami- 

 nar like the Pinna, and many are remarkable for the beau- 

 tifully furbelowed scales which they develope, and for the 

 exquisitely festooned and serrated character of the epider- 

 mis. The external stria; are often deposited like concen- 

 tric silken ridges. 



Many of the Avicula secrete pnaris iu addition to the 

 well-known A. luargaritifira . Mr. Cuming possesses fine 



pearls taken from the A. macroplna, and some of a jjccu- 

 liarly delicate warm hue produced by the A. pica. 



The Avicula are abundant in the tropical and sub-tem- 

 perate seas, and are handsomely represented on our own 

 shores by the A. Tarenlina of the Mediterranean. The 

 smaller species, some of which are extremely fragile, attach 

 themselves chiefly to bunches of Zoophytes. 



Species L (Fig. 1 and 21, Mus. (Juming.) 

 Avicula margaritifeea. Avic. testa orbiculari, plano- 

 convexd, superne recta, postice subsinuatd, ubique con- 

 centrice laminatd, laminis versus marginem in seriebm 

 longitudinalibus squamarmn prodmtis, squamis plus 

 minus irregulariter contortis; grkeo-oUvaced, prope 

 uiubonem virescenle. 

 The pearly Avicula. Shell orbicular, flatly convex, 

 straight at the upper part, slightly sinuated on the 

 posterior side, everywhere concentrically laminated, 

 lamina; produced towards the margin into longitu- 

 dinal rows of scales, scales more or less irreonlarlv 

 twisted ; greyish-olive, green near the umbo. 

 Mytilus margariti/er, Limia;us, Syst. Nat. p. 1153. 

 Margarita Sinensis, Leach. 

 Avicula margarilifera, De Roissy. 

 Meleagrina mxirgaritiftra, Lamarck. 

 Avicula meleagrina, De Blainville. 

 Avicula radiata, Leach. 

 Hab. Tropical and sub-tropical seas iu both hemispheres. 

 Of this colossal species, the well-known Pearl Oyster 

 of the South Seas, we have figured a young specimen at 

 Plate VIII., to show the nature of its sculpture in an early 

 stage of growth. As the shell advances in age, and the 

 valves thicken, the sculpture becomes obsolete. 



