94 AVICULID^E. 



SHELL shaped like the animal, inequilateral, scaly outside 

 and nacreous within: beaks straight: ligament long and 

 narrow, mostly internal and contained in a groove. 



In this distinct family we have only two genera, each 

 containing but a single species, which widely differs 

 from the other. The intermediate links are wanting at 

 our northern end of the chain. Elsewhere both genera 

 and species abound in great variety; and, in remote 

 periods of the earth's history, what are now called the 

 British seas had also their full share. In studying the 

 particular fauna of any district we are too apt to regard 

 it in an isolated point of view, instead of associating it 

 with the faunae of other and distant regions. No mem- 

 ber of this large family has been noticed as inhabiting 

 the north-east coast or the northern part of the western 

 coast of America, nor is any one enumerated in the 

 lists of Arctic or Scandinavian Mollusca. The shell is 

 composed of outer and inner layers. The outer layers 

 are of a fibrous texture, and consist of extremely minute 

 and closely-packed tubes or cells, which exhibit on 

 their surface irregularly hexagonal prisms; they are 

 separated from each other by a deposit of unusually 

 thick animal matter ; and, upon being steeped for some 

 time in caustic potash, they easily become disintegrated 

 and fall asunder, resembling in that state extremely 

 short threads of spun glass. The inner layers are more 

 compact and highly iridescent. The surface of the 

 shell, both in Avicula and Pinna, appears under the 

 microscope to be finely punctured, as in some species of 

 Lepton. The anterior adductor muscle is small, showing 

 a transition from the Monomyaria to the Dimyaria. 

 Leach included this family in the first-mentioned di- 

 vision. 



