GASTROCH^NIDiE. 89 



it is beaded lengthwise, and lias a more square outline 

 with rounded angles. Philippi calls it "rotundato- 

 quadrangula/^ a definition at which mathematicians 

 might be inclined to smile. 



Family XXII. GASTKOCH^'NID^, Gray. 



Body oblong: mantle large and thick: tubes long, united 

 throughout ; orifices cirrous : gills unequal, prolonged into the 

 branchial tube : palps small, somewhat triangular and thick : 

 foot finger-shaped, sometimes byssiferous. 



Shell enclosed in a testaceous flask-like case with a narrow 

 bipartite or divided neck ; it is oblong, equivalve, very inequi- 

 lateral, wedge-shaped and widely gaping in front ; anterior 

 end pointed : epidermis membranous : heahs nearly terminal : 

 ligament long, external : hinge straight, toothless but furnished 

 with a horizontal plate or shelf-like process : pallial scar broad, 

 and deeply sinuated : muscular scars stroug. 



This family connects the SaxicavidcB and Pholadida. 

 Its smooth shell is not unlike that of S. rugosa var. 

 pholadis, which has also a ventral gape, and is most 

 frequently toothless ; although its reflected hinge-plate 

 and pointed anterior extremity remind us of Pholas 

 crispata. The foot is more characteristic of the first- 

 named family, and the pallial tubes of the latter. But 

 its distinctive and peculiar feature is the outer case or 

 covering, in which all the Gastrochoinidcs enwrap them- 

 selves on arriving at maturity. The body of this case 

 appears to be formed, like the shell, by a secretion of 

 the mantle ; and the enormous opening in front, between 

 the valves, must greatly facilitate the work. The animal 

 probably uses its flexible foot, turning it round on every 

 side, to mould the walls of the case. There can be no 

 doubt that the neck is elaborated by the siphons. 



