PHOLAS. 121 



The shape is somewhat conoid, being broad, and rounded in 

 front, and strongly tapering to a more or less obtuse point poste- 

 riorly. The valves are similarly compressed behind, but ventri- 

 cose, or even tumid anteriorly ; their texture is very thin and 

 fragile, and under a more or less distinctly visible yellow epider- 

 mis, they are of a dull white. The surface is vertically divided 

 a little before the middle by an extreme and abrupt difference of 

 sculpture ; the posterior area being distinguished by concentric 

 elevated plicae, which become more or less obsolete as they recede 

 from the beaks and the line of separation, where they are strongly 

 indicated ; whilst the anterior portion is more or less closely 

 covered with rather strong oblique laminar crenulated stria?, 

 which form distinct angles towards their middle, at which point 

 the foremost ones diverge, and the following ones converge. A 

 thin, smooth, tumid, triangular space near the front ventral margin, 

 covers in the adult the vast gape which is there present in the 

 immature examples. The extremity of the posterior side, which 

 is greatly the longer, is slightly hiant, and the ventral edges are 

 prevented approaching each other by an elongated testaceous 

 accessory plate, which lies between their hinder edges. There is 

 a kind of double appressed reflection of the front dorsal edge over 

 the umbones, which supports the large and somewhat heart-shaped 

 shield which entirely conceals them. This latter, which is rather 

 blunted in front, and profoundly incurved behind, is followed by 

 a third accessory testaceous plate, which separates the posterior 

 dorsal margins, and is elongated and narrow in shape. The sub- 

 umbonal toothlike apophysis is flattened but slender ; and there 

 is a deep umbilicus-like impression in front of the beak, caused 

 by the primary and more abrupt reflection of the dorsal margin. 



At the first glance this species bears a strong resemblance to 

 the P. cuneiformis (Journ. Ac. Sci. Philad. vol. ii. p. 322. — Dekay, 

 New York Moll. p. 248) of Say, which, not being very clearly 

 characterized by its author, seems but little known in England ; 

 it may, however, readily be distinguished by the acuminated 

 hinder termination of its smaller and more solid umbonal shield, 

 and in all stages of growth by the non-angulation of its elevated 

 anterior striae. 



The specimens taken in England rarely exceed an inch in 

 length, and even the more strictly exotic ones are not greatly 

 larger. They peculiarly affect mahogany. 



VOL. I. R 



