250 



AVICULACE^. 



In this family, of which the British examples are few 

 and rare, we have a passage from the dimyarian to the 

 monomyarian lamellibranchiate bivalves, great groups, 

 which though convenient sections, can scarcely be re- 

 garded as of ordinal value. Avicula has usually been 

 included in the latter, Pinna in the former section, yet 

 not only are their animals very similar, but also their 

 shells ; for though Pinna appears of a very different 

 shape from Avicula when placed with its truncated ex- 

 tremity towards us, and its pointed beaks from us, 

 yet that is not its natural position for comparison, since 

 it is really an extremely oblique shell, and therefore 

 should be placed before us in the direction of greatest 

 dimension. The close affinity of these shells is further 

 proved by their microscopic structure, as made known 

 by Dr. Carpenter, who first maintained their relationship, 

 associating in the same family the genera Perna, Malleus, 

 Crenatula, Vulsella, Gervillia, and Inoceramus. All these 

 genera have the exterior of the shells composed " of 

 prismatic cellular substance, and the interior of true 

 nacre." The size of the cells varies greatly in the dif- 

 ferent groups. 



The animals of all the Aviculacea have freely open 

 mantles, and a small foot with a powerful byssiferous 

 gland. 



