68 MOLLUSCA FROM THE GREAT OOLITE. 



The lateral diameter, in regard to the height, is as eleven to nine ; the umbones are 

 nearly mesial, and pointed, but depressed ; the concentric strise are very slightly impressed, 

 which gives to the shell a smooth and depressed aspect. It is not very common, but 

 occurs in more than one of the shelly beds of the Great Oolite. 



Localities Minchinhampton and Bisley Commons. 



LuciNA CRASSA, Sow. var. Tab. VI, fig. 13. 



LuciNA CRASSA, Sow. Min. Con., t. 557, f. 3, 1827. 



Testa crassd, sub orbicular i, plano-convexd, lateribus subaqualibm ; umbonibus acutis 

 medianis, cardine marginali recto, obliquo declivi ; lunuld parvd obliqud ; basi arcuatd, 

 lineis concentricis crebris irregularibus. 



Shell convex, suborbicular, the sides nearly equal ; umbones acute, mesial ; hinge 

 margin straight, oblique, and sloping; lunule small, oblique; base regularly rounded, 

 concentric lines closely arranged and irregular. 



The umbones are mesial and curved forwards, so that the anterior side of the shell is 

 less produced than is usual with the genus ; the degree of convexity near the umbones is 

 moderate, and less than in L. obligua. It occurs very rarely well preserved in the 

 planking beds of the Great Oolite. 



The specimen figured is contained in the collection of the British Museum. 



Worn specimens of this species, of which the shell has become thin, and the exterior 

 markings obliterated, are difficult to distinguish from what we consider a distinct species, 

 L. rotundata, and which may prove to be only a variety. 



Locality. Minchinhampton Common. 



LuciNA ROTUNDATA, Boemer, sp. Tab. VI, figs. 14, 14 a. 



TAsTARTE ROTUNDATA, Roemer. Vers. Oolith., t. 6, f. 12, 1836. 



Testa subtransversd, incequilaterd, oblique orbiculari, concentrice lineatd, convexd, antice 

 subproductd, complanatd ; umbonibus parvis incurvis. 



Shell somewhat transverse, inequilateral, obliquely orbicular, concentrically lineated 

 convex, anterior side rather produced ; umbones small, incurved. 



Specimens vary both in the degree of convexity, and in the proportions between the 

 lateral diameter and the height, but the former measurement always exceeds the latter. 

 The concentric lines or plications are very irregular and faintly marked, so as to give 

 a general smoothness to the surface. 



It occurs somewhat rarely in the shelly beds of the Great OoHte, and has also been 

 recognised in the upper ragstones of the Inferior Oolite. 



Localities. Minchinhampton Common in the Great Oolite ; Rodborough Hill in the 

 Inferior Oolite. 



