BIVALVIA. 37 



Our Great Oolite specimens must be considered as a variety of this well-known shell, 

 they are much attenuated at the anterior extremity, and unusually expanded and com- 

 pressed at the posterior extremity. It occurs rarely in the Stonesfield Slate, and has not 

 been observed in the shelly beds of the Great Oolite. 



Localities. In the slaty beds at Minchinhampton Common ; Stonesfield, Oxfordshire. 

 Also in the Upper Marly deposits of the Great Oolite at Felinersham, Bedfordshire, and 

 Blisworth, Northamptonshire. 



M. D'Orbigny has changed the generally known name given to this species by Mr. 

 Sowerby, as Gmelin had previously described a recent species under the name of 

 Mytilus plicatus. 



Mytilus (Modiola) tenuistriatds, Munst. Tab. IV, fig. 6. 



Mytilus tencistriatus, Gohlfuss. Petref., t. 131, f. 5, 1840. 



— — Buekman, Geol. Cheltenham, p. 69, t. 3, f. 3. 



Testa ovato-cordatd, fornicato tenuissime concentrice striata, umbonibus terminalibus 

 arcuatis, margine cardinali recto, brevissimo, dorso alto angusto, latere inferiore cordato, 

 antice subventricoso. (Gohlfuss.) 



Shell ovately cordate, short, very convex ; umbones terminal acute and curved forwards, 

 hinge border straight and short, dorsal surface very much elevated and narrow, inferior 

 border sinuous, its anterior portion rather ventricose ; concentric striations regular, fine, and 

 closely arranged. 



This is a short, obtuse shell, and the convexity of the valves is so considerable that the 

 diameter through them, when in apposition, is nearly equal to their length ; the striations 

 are very delicate, and the test thin. 



Locality. For this pretty species we are indebted to the kindness of Professor 

 James Buekman, who collected it in a bed of yellow Clay at Sevenhampton, together 

 with numerous other testaceous relics, which though numerous in species, are for the 

 most part badly preserved. It is probable that this stratum, which can be traced for 

 two miles and upwards, is a subordinate local deposit of the Great Oolite ; at Seven- 

 hampton, it is situated 22 feet above the Stonesfield Slate. 



Mytilus tumidcs. Tab. IV, fig. 5. 



Testa arcuatd, injlatd, striata concentrice rugosd, margine cardinali curvato, margine 

 inferiore concavo, umbonibus terminalibiis obtusis, crassis. 



Shell curved, inflated, with rugose concentric striae, hinge margin curved, inferior 

 margin arched or concave, umbones terminal, obtuse, and thick. 



The dorsal surface is very elevated, narrow anteriorly, and more flattened posteriorly; 

 the convexity of the valves is so considerable that the diameter through both is equal to 



