PHOLAS. 105 



open and exhibiting an oval gape towards the front, whence 

 there is a regular slope both above and below to form the 

 posterior end, which is rounded, and has a sharp edge, with a 

 decided gape ; dorsal margin on the anterior side short and 

 obliquely convex: beaks very near the anterior end: hinge- 

 line flexuous : hinge-plate extremely broad ; it forms a double 

 fold, one of which has a free cutting edge and projects out- 

 side in the middle of the hinge-plate, and the other adheres 

 for the most part to the anterior side, its outer edge being 

 likewise free ; the interspace between these folds is fitted with 

 about a dozen transverse plates, besides occasionally a few 

 short intermediate processes in the opposite direction ; the 

 hinge-plate is sometimes crossed in its thickest part by two or 

 three oblique tooth-like ridges : apophyses strong, broad, and 

 curved, concave and expanding outwards : dorsal shields, two 

 on the anterior side, large, irregularly lance-shaped, broader 

 in the line of the beaks, and often cracked in a direction 

 radiating from outside ; another in the middle is morticed into 

 the two anterior shields, and is of an irregularly triangular 

 shape, twisted, and very solid, lying perpendicularly across the 

 valves; the fourth or posterior shield is long, narrow, and 

 slightly bent, so as to fit the slope of the shell on that side. 

 L. 1-75. B. 5. 



Yar. 1. gracilis. Shell smaller, more slender, and of a 

 finer and thinner texture. 



Yar. 2. decurtata. Shell stunted or truncated at the pos- 

 terior end, and of a coarser and more solid texture ; sculpture 

 closer and usually effaced. 



Habitat : Slate rocks, coal-shale, new-red sandstone, 

 chalk, marl, peat, and submarine wood in Guernsey, 

 the south of England, and Bristol Channel ; Seacombe, 

 Lancashire (Dr. Walker) ; north, east, and south of 

 Ireland. Var. 1. At extremely low tides below the 

 Warren, Exmouth, in pure sand (Clark). Var. 2. Oc- 

 casionally met with in hard rocks. Fossil at Belfast 

 (Grainger) ; Sussex (Godwin- Austen) ; in the Scotch 

 glacial beds at Ayr and Stevenston (J. Smith and 

 Landsborough) ; Tarento (Philippi) : and the variety 



F O 



