244 MOLLUSCA FROM THE CRAG. 
This species is not very abundant in the Coralline Crag; but in the native 
bed at Walton Naze in the Red Crag, specimens are met with in large numbers, 
although somewhat 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 probably 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, deeply 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. Macrra artopta, S. Wood. Tab. XXIII, fig. 4, a, 0. 
Spec. Char. Testa trigonuld vel obtuse cuneatd ; tumidiusculd, inequilaterali, antice 
rotundatd, margine dorsali postico rectiusculo ; umbonibus subprominulis : dentibus lateralibus 
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, 25 inches. Height, 2 inches. 
Locality. Cor. Crag, Sudbourne. 
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 be considered as belonging to a distinct species, differing from JZ. 
arcuata, which is perhaps its nearest relative, in having the dorsal margin 
less curved than in any specimens of that species which have come under 
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 47. arcuata, from the same 
Formation, were found at Ramsholt. 
5. Mactra procrassa, 8. Wood. Tab. XXIV, fig. 8, a, 3b. 
Spec. Char. Testd magna, crassé; irregulariter striata, ovato-trigonuld, inequi- 
laterali ; anticé sub-angulatd, postice rotundata ; JSoved ligamenti magna ; dentibus lateral- 
bus perpendiculariter striatis. 
