494 



MANUAL or THE MOLLUSC A. 



Shell oblong, gaping at each end ; posterior side shortest ;' 

 ligament large and prominent ; epidermis black, extending 

 beyond the margins ; anterior muscular scar long, pallial im- 

 pression irregular, slightly sinuated. 



Animal larger than its shell, sub -cylindrical ; mantle closed, 

 siphons united, protected by a thick envelope ; orifices small ; 



Fig. 268. Glycimeris siliqua, Chemn. Newfoundland. 



a, a, adductor muscle ; p, pedal muscle ; s, siphonal muscle ; /, foot; t, labial 



tentacles ; g, gills, much contracted and crumpled. 



pedal opening small anterior ; foot conical ; palpi large, striated 

 inside, the posterior border plain ; gills large, extending into 

 the branchial siphon. 



Distribution, 2 species. Arctic Seas, Cape Pariy, North 

 Western America, Newfoundland. 



Fossil, Pliocene — . Britain, Belgium. 



Family XIX. — Anatinid^. 



Shell often inequivalve, thin ; interior nacreous ; surface 

 granular ; ligament external, thin ; cartilage internal, placed 

 in corresponding pits and furnished with a free ossicle; 

 muscular impressions faint, the anterior elongated ; pallial 

 line usually sinuated. 



Animal with mantle-margins united ; siphons long, more or 

 less united, fringed ; gills single on each side, the outer lamina 

 prolonged dorsally beyond the line of attachment. 



Pholadomya and its fossil allies have an external ligament 

 only ; has no ossicle. The external surface of these shells is 

 often rough with large calcareous cells, sometimes ranged in 

 lines, and covered by the epidermis ; the outer layer consists of 

 polygonal cells, more or less sharply defined ; the inner layer is 

 nacreous. 



