Counties of Durham and Northumberland. 45 



occurs in casts, on which all the permanent structures of the 

 interior of the shell are faithfully impressed. One can, by a 

 study of these casts alone, restore and represent all the im- 

 portant characters of the interior; but in addition to these 

 casts, one frequently finds at Tunstall the shell itself, showing 

 the interior as perfectly preserved as in recent shells. It is 

 therefore rather a matter of surprise that no better figures of the 

 interior of this shell have yet been published. Mr. King figures a 

 gutta-percha cast of the interior of one valve only, and that one, 

 judging from the specimens that I have examined, very incor- 

 rectly, as will best be proved by comparing his figure, Mon. pi. 1 1. 

 fig. 10, with Mr. Davidson's Mon., pi. 4. figs. 19, 21, and with 

 good casts from Humbletou or interiors from Tunstall. The most 

 incorrect part of Mr. King's restoration is the strongly granu- 

 lated or obscurely dendritic appearance of the adductor muscular 

 impressions, which are also erroneously divided into two sets ; 

 and the reniform impressions are made to take their origin in 

 these more distinctly than is warranted by good specimens. 

 The cardinal boss, the hinge-margin, and the spine-like callo- 

 sities on the inner surface of the shell are also all very imper- 

 fectly represented. Dr. Geinitz's figures of the same valve are 

 more correct than the one already mentioned, but the adductor 

 muscular impressions are too leaf-like and lobed. All the other 

 figures of this valve that have been published since 1850 appear 

 to be merely copies from King's. 



The hinge-line of the upper valve is not quite straight, but 

 slightly angulated, the angle being strongest near the boss. 

 The boss, or cardinal muscular fulcrum, varies slightly in form, 

 but it is generally bifid at the extremity, each part being slit by 

 a deep triangular groove or furrow. When in situ it fills nearly 

 the whole of the umbonal cavity of the lower valve and presses 

 against its inner surface. It may thus assist in keeping the valves 

 in position. About the base of the boss the shell is very much 

 thickened, and from it a strongish ridge runs along on each side 

 parallel to the hinge-margin, which gives to the latter a bevelled 

 appearance. On the outer side of this ridge is seen the row of 

 depressions caused by the cardinal spines. From the base of the 

 boss a thin plate or septum proceeds straight forwards into the 

 cavity of the shell, becoming deeper and free in front ; it sepa- 

 rates the adductor muscular impressions and the reniform callo- 

 sities, to which latter the oral arms were probably attached. The 

 muscular impressions on this valve have each a triangular form, 

 and as they are placed close to each side of the median plate, 

 they have together a fan-shaped form. They are considerably 

 raised, and most so on the anterior margin. Their surface is 

 sculptured out into deep curved linear hollows for the attach- 



