THOLAS. 115 



SO excessively gaping at both ends that the valves only 

 touch each other at the hinge and in the middle of the op- 

 posite margin. The surface is rather obliquely divided 

 into two nearly equal portions by a broad groove-like chan- 

 nel which, proceeding from the umbones to the middle of 

 the extremely-arcuated ventral margin, is internally indi- 

 cated by a corresponding costa or rib-like elevation. The 

 anterior portion, which is eminently tumid, is covered with 

 rather close-set concentric laminar striae, whose free edge 

 is less closely dentated in radiating rows, which do not 

 quite extend to the dividing channel, posterior to which 

 the shell is flatter, and only marked with the concentric 

 wrinkles of increase. The anterior extremity forms a very 

 short obtuse beak, and the posterior termination broadly 

 rounded. The dorsal edges, which are but little sloping, 

 are incurved near the hinge. The hinge-margin is broadly 

 reflected, simple, and unprovided with denticles. The sub- 

 umbonal tooth is long, flat, subspatulate, but little concave 

 at its extremity, and much curved. The single testaceous 

 accessory plate is small, but soljd, and is situated at the 

 end of the skin which covers the front dorsal gape ; it ap- 

 pears trigonal when imbedded, the sides being nearly 

 straight, and meeting in a rectangle, whose apex forms the 

 posterior termination of the plate, but when detached from 

 the shell more nearly approaches the form of an arrow-head, 

 the concealed underlapping portion constituting very 

 elongated barbs. The animal is dusky-white, its siphon 

 tinged with brown and rough with papillse. 



Our largest examples exceed three inches in length by 

 about one and three quarters in breadth. On the southern 

 shores of England it is reckoned among the less common 

 shells, but is occasionally found in smaller numbers imbed- 

 ded along with the other Pholades at the Reculvers (S. H.), 



