42 MOLLUSCA FROM THE GREAT OOLITE. 



The hinge border is curved, and equal to half the entire length of the shell, the 

 terminal umbones are remarkably acute, without any expansion upon the lower side. It 

 is tolerably abundant in the shelly beds of the Great Oolite. 



Dimensions of a medium sized specimen : Length, 23 lines ; height, 12 lines ; diameter, 

 through both the valves, 9 lines. 



Locality. The whole of the Minchinhampton district ; Felmersham, Blisworth, &c. ; 

 in the marly deposits belonging to the upper portion of the Great Oolite. 



Mytilus Binfieldi. Tab. IV, fig. 10. 



Testa ovatd suiarcuatd concentrice striata, umhonihus obtmis, dorso fornicato, marline 

 cardi7iali recto, latere inferiore abnipto antice convexo. 



Shell ovate, superior side compressed, inferior side elevated, with terminal obtuse 

 umbones, hinge margin rather oblique, the greatest convexity is about the middle of the 

 valves, the lower margin is straight, and the general aspect of the shell is smooth. 



It occurs rarely in the shelly beds of the Great OoHte. 



Locality. Minchinhampton Common. 



Named in compliment to Mr. W. R. Binfield, who has assiduously collected the fossils 

 of the Oolite. 



LiTHODOMUs, Cuvier, 1817. 



Gen. Char. Shell elongated, subcylindrical, anterior extremity rounded and convex, 

 posterior extremity more attenuated, margins of the valves close all round ; umbones 

 anterior, terminal, pointed ; hinge without teeth, ligament internal linear, and placed in a 

 lengthened groove. 



Lithodomus perforations are extremely common in the shelly beds of the Great Oolite ; 

 they are not, however, confined to the surface of any particular bed, but occur indifferently 

 throughout a considerable thickness of shelly rock, and the more massive bivalves often 

 exhibit their perforations, more especially Trichites and Astarte subquadrata ; the number 

 of these perforations proves that the Lithodomi existed in great profusion, and contrasts 

 singularly with the paucity of the specimens preserved ; we may infer from this fact, that 

 a very qualified degree of dependence should be placed in the number preserved of certain 

 thin and fragile bivalves, as representing the actual number of individuals which existed in 

 the seas of the Oolitic period. The valves which occur in the shelly detritus are well 

 preserved, others which remained in the hollow oval crypts are uniformly very tender and 

 imperfect, they can scarcely be said to be fossilized ; the crypts themselves are for the 

 most part empty, or contain only a little hardened mud ; when, however, the crypts are 

 filled with calcareous spar, it is probable that the living animals themselves were 

 entombed in the deposit. 



