FAMILY 3. AVICULACEA. 143 



Family 3. AVICULACEA. 



Testa irregularis, tenuis, fibro-laminaris, extus interdum foliata. Cardo 

 edentulus ligamento variabili, in fossula unica aut pluribus inserto. 

 Impressio muscularis interdum eomposita. Animal bysso affixum. 



Lamarck originally included the whole of the byssus-bearing mollusks 

 in one family, Les Byssiferes ; but upon noting the variety of forms 

 that so extensive a series naturally included, he instituted the subdivision 

 that has been since adopted. The Aviculacea are, perhaps, more variable 

 in their general characters than any of the byssiferous Tropiopoda, and 

 have yet undergone less change in their generic distribution from sub- 

 sequent authors. They correspond entirely with Lamarck's family of 

 Les Malleace'es, except in the introduction of the genus Vulsella, which 

 he included with the Ostracea ; the Vulsella were removed by Cuvier on 

 account of their affinity with the Mallei ; the hinge is very similar, and 

 they moreover differ from the Ostrece in being provided with a byssus, 

 and therefore in not fixing themselves immediately by their shell. In 

 this form the family of the Aviculacea is adopted by most authors : by 

 De Blainville under the title of Margaritacea ; by Latreille under that of 

 Oxygona ; and by De Ferussac and Gray under that which w T e have here 

 adopted. They come with great propriety after the Mytilacea, because 

 their shells have the same peculiar fibro-laminal structure as those of the 

 genus Pinna ; they however differ from the Pinna both in the position of 

 the byssus, which passes out through a notch near the hinge, and in that 

 of the ligament, which is inserted in one or more pits on the hinge mar- 

 gin. These ligamental pits vary considerably both in their shape and 

 number, and chiefly serve to fix the subdivision of the family into genera. 

 In some the hinge is characterized as having only a single central pit for 

 the insertion of the ligament ; in others it consists of a series of small 

 concavities ; and in others, again, of a series of parallel grooves. 



