108 MOLLUSCA FROM THE GREAT OOLITE. 



Collectors have very generally mistaken this species for Ceromya excentrica, a shell which 

 is stated to occur abundantly in the upper or Portlanclian Oolite of Switzerland, at Porrentroy, 

 and in a similar parallel in the Jura of Soleure ; C. plicata has not heretofore been 

 adequately figured or described ; the specimens figured by M. Agassiz represent adult and 

 even aged shells, not well preserved, and in which the V-hke angle of the ridges has 

 nearly disappeared ; his description is likewise more than usually meagre, and, in the 

 absence of other evidence, the reader would be inclined to believe that the author had 

 unnecessarily separated this shell from C. excentrica, but an examination of specimens in 

 several stages of growth has convinced us of the propriety of the specific distinctions which 

 are given in the ' Etudes Critiques ;' the general figure is near to C. excentrica, except that 

 in the adult forms the superior border is more compressed and elevated, and the posterior 

 aperture is much larger ; the change in the direction of the ridges upon the surface is not 

 peculiar to C. excentrica, but occurs in other species of the same genus, neither is it a 

 regular and constant feature in any species, or rather, we should say, that it is never found 

 in the young condition of any species. All the specimens known are casts, the delicate 

 and very perfect markings in young examples is a sufficient indication that the test must 

 have been of extreme tenuity, and the partial obliteration of these features with advance of 

 growth, evidences a corresponding change in the character of the test. In the specimen 

 figured by Agassiz the angles of the reflected ridges are less acute. 



Dimensions. Our largest specimen is in length 3 \ inches ; in height, 2| inches ; 

 the diameter through both the valves being 2| inches. 



Localities and position. We have observed this species in the upper beds of the 

 Inferior Oolite in Gloucestershire jn the fuller's earth it has occurred over the Sapperton 

 tunnel of the railway, from which deposit a specimen has kindly been forwarded to us by 

 John Wilson, Esq., of Gloucester ; we have ourselves obtained it from certain hard lime- 

 stone beds near to the base of the Great Oolite in the Minchinhampton district, and 

 Professor Buckman has recorded a specimen which he obtained in a bed of clay at 

 Sevenhampton, which appears to be a little higher in the series ; it is, however, rare 

 at each of these localities. 



CEROMYA CONCENTRICA, Sow., sp. Tab. X, fig. 8a, b. 



ISOCARDIA CONCENTRICA, Sow. Min. Con., tab. 491, fig. 1. 



Phil. Geol. York., 1, pi. 1 1, fig. 40. 



Morris. Catal. Brit. Foss., 1st. Ed. p. 88. 1843. 



Testa ventricosd, ovato-obliqud, umbonibus magnis incurvis subanticis, latere antico 

 convcxo,postico subcompresso, basi curvato, lateribus fastiffiis tenuibus concentricis regularibus 

 crebris. 



Shell ventricose, ovately oblong ; umbones large, incurved, anterior to the middle of 



