62 



Family 3. AVICULACEA. 



Shell ; irregular, rather thin, fibro-laminar ; hinge toothless ; liga- 

 ment marginal, either linear or enclosed in pits or grooves ; 

 lower valve notched or si nuated for the passage of a byssus. 



The Avictdacea, of which the large Pearl Oyster is the type, have shells 

 of a fibrous laminar structure, partaking of the foliated structure of the 

 Oysters proper; but the animal has a well-developed foot, such as appears 

 only occasionally in the preceding family in a rudimentary form, and it is 

 furnished with a gland which secretes a byssus of considerable strength. 

 Bivalves that are provided with the means of mooring themselves to foreign 

 bodies, have no interlocking teeth, except in the ponderous Clams. The 

 hinge of the Avicidacea is only a ligament hinge, the ligament being more 

 than usually developed, and either linear or contained in variously arranged 

 pits or grooves according to the genus. 



The shells of this family have, of all bivalves, the least proportion of 

 lime in their composition, and the largest, therefore, of membranous tissue ; 

 and the testaceous fluid used for the internal lining is that beautifully iri- 

 descent silvery substance called nacre, or mother-o'-pearl. The genera are — 



Avicula. Vulsella. Malleus. 



Perna. Crenatula. 



Genus 1. AVICULA, Klein. 



Animal ; oval, flattened, with the lobes of the mantle free through- 

 out, frilled at their edges ; foot sometimes small, sometimes 

 rather long, furnished with a byssal groove ; mouth and labial 

 palps rather large, obliquely truncated, adductor muscle strength- 

 ened by smaller auxiliary muscles. 



Shell ; inequivalve, sometimes orbicular, sometimes obliquely tri- 

 angular, ivith the anterior side more or less elongatcly winged ; 

 hinge toothless, but furnished in most instances, beneath the 

 umboes, with an obscure tubercle ; ligament linear, more or less 

 dilated in the middle. 



The obliquely triangular winged Avicula, or Swallows, and the round 

 Avicula, or Pearl Oysters (the Meleagrina of Lamarck), are associated to- 

 gether in this genus. Though differing materially in shape, the animal is 

 the same, and even the roundest form of Meleagnna possesses in a rudi- 

 mentary shape the winged process which is developed in so extraordinary 



