152 CLASS I. TROPIOPODA. ORDER II. UNIMUSCULOSA. 



(Class III. Gasteropoda. Family 3. Macrostomata) ; in these animals, how- 

 ever, they are but rarely to be met with ; and as their nacre is either opake 

 or less iridescent, they are only preserved as curiosities. Pearls, therefore, 

 are produced by a disease of the nacreous glands, resulting either from 

 accident, or from a superfluous discharge of the nacreous matter, which 

 is more or less common to other mollusks, particularly the Unimuscular 

 Tropiopoda ; and if the irritating process, before described, were similarly 

 applied to other species, similar results would no doubt be produced. 

 We refer our readers to the many published accounts of the Pearl Fish- 

 eries in the Indian and Chinese seas for a full history of the operations 

 of the divers, &c. 



The shell of Avicula may be described as being irregular, fibro-laminar, 

 sometimes forming beautifully imbricated scales, inequivalve, transverse, 

 and straight at the base ; the sides are sometimes short, the anterior 

 often very long. The byssus passes out through a notch at the base of 

 the left valve. The hinge is edentulate, but in most species an indication 

 of teeth is offered by the appearance of a dentiform tubercle just beneath 

 the umbo in each valve. The dorsal area of the valves which bears the 

 ligament is marginal, linear, narrow, and somewhat dilated in the middle. 

 The adductor muscle appears to be of compound construction, as there is 

 generally an interrupted series of small marks, showing the attachment of 

 certain accessory cartilages running from the large central impression 

 down towards the umbones. 



Examples. 

 PI. CIX. Fig. 1 . 

 Avicula heteroptera, Lamarck, Anim. sans vert., new edit., vol. vii. 

 p. 98. Sowerby, Genera of Shells, No. 14. 



PI. CIX. Fig. 2. 

 Avicula aculeata, Sowerby, Genera of Shells, No 14. 



PI. CX. 



Avicula margaritifera, Bruguiere, Enc. Meth., pi. 177. f. 144. Sow- 



