244 I^IOLLUSCA FROM THE CRAG. 



This species is not very abundant in the Coralline Crag; but in the native 

 bed at Walton Naze in the Red Cra<i:, specimens arc met with in large numbers, 

 although sonunvhat difficult to obtain perfect, owing to their extreme fragility. The 

 exterior is always more or less ridged, from the unequal degree of decomposition, and 

 the surface is never in its original condition; but, judging from those in which it is 

 partially preserved, it was proliably nearly smooth in the living state. 



The ligamental area is large and simple, running close up to the umbo ; and the 

 cardinal A-formed tooth very small in the left valve, with a corresponding cavity in the 

 right ; the lateral teeth are double in the right valve, dcc])ly ridged on both sides of 

 the interior ; the lateral teeth of the left valve are single, and ridged on the outside ; 

 the muscular impressions are large, not very deep seated, and the sinus in the mantle- 

 mark is somewhat narrow, extending into the interior at least one third of its entire 

 length, but variable, being modified by the comparative length of the shell. 



4. Mactr.v autopta, S. IFood. Tab. XXIII, lig. 4, a, b. 



Spec. Char. Testa trigomdd vel obtuse cuneatd ; tumidiusculd, inaquUaterali, atitice 

 rotimdatd, marr/hie dorsali postico rectiusculo ; umbonihis subprominulis : de7itibHS laferalibus 

 perpendiculariter striatis. 



Shell trigonal or obtusely wedge-shaped, slightly tumid, inequilateral, anterior side 

 rounded, dorsal margin of posterior side somewhat straight, umbones rather promi- 

 nent, lateral teeth striated perpendicularly. 



Length, 1\ inches. Height, 2 inches. 



Locality. Cor. Crag, Sudboume. 



Several specimens of a Mactra, contained in the collections of Mrs. Corder, Mr. 

 Perry, and my own, appear to possess a uniformity of character, such as I think 

 entitle them to l)e considered as belonging to a distinct species, differing from M. 

 arcuata, which is perhaps its nearest relative, in having the dorsal margin 

 less curved than in any specimens of that species whicli have come nnder 

 my inspection. This shell is also rather more tumid, and it has a more elevated 

 umbo, and its whole aspect appears different. I have seen it only from the Coralline 

 Crag, in the neighbourhood of Orford. My specimens of M. arcuata, from the same 

 Formation, were found at Ramsholt. 



5. Mactra procrassa, .9. Wood. Tab. XXIV, fig. 8, a, b. 



Spec. Char. Tcufd magna, crassd ; irregiilariter striata^ ovato-trigonuld, inaqui- 

 laterali ; antice sub-angulatd, postice rotu7idafd ; fovea ligamenti magna ; dentibus laterali- 

 btis perpendiculariter striatis. 



