BIVALVIA. 109 



the valves ; anterior side convex ; posterior side more elongated and compressed ; base 

 curved ; the sides of the shell with regular closely arranged concentric and fine ridges. 



The umbones are prominent and elevated, more especially by comparison with CI Symondsii 

 and C. Northamptoniensis, the two contemporaneous forms which most nearly approach to 

 it; owing to ihis prominence, the superior border is rendered slightly concave. The valves 

 fit closely, except at the posterior extremity, which has a slight aperture. The test is 

 never preserved. It is liable to be confounded with a larger and magnificent Inferior 

 Oolite species, which occurs in .the neighbourhood of Stroud, and has the test preserved; 

 this latter, which we believe to be the Ceromya Bajociana of D'Orbigny, ' Prodrome 

 de Paleontologie,' p. 275, and, probably, the Isocardia concentrica of Phillips ; in this shell 

 the umbones are very large, and curve gracefully forwards ; they are more median and less 

 oblique ; the general form is more ventricose, and the posterior side is shorter than in the 

 true Ceromya concentrica. 



Ceromya concentrica does not occur in the shelly beds of the Great Oolite, it occurs in 

 the upper portion of the formation associated with C. Symondsii in the Minchinhampton 

 district, and also near to Nymphsfield, it is also abundant in the Marl bed of the Inferior 

 Oolite, and in the upper division of the same formation. 



Dimensions of a Great Oolite specimen. Height, 16 lines; length, 20 lines ; diameter 

 through both the valves, 14 lines. 



Localities. The neighbourhood to the east of Minchinhampton, and at Nymphsfield, in 

 the Great Oolite; the escarpment of the Cotteswolds generally in the Inferior Oolite. 



Ceromya similis. Tab. XII, fig. 12. 



Ceromya similis, Lycett. Geol. Soc. Journ., 1853, p. 340, pi. 14, fig. 2. 



Testa ovato-oblonyd, convexd ; umbonibus anticis incurvis ; latere antico brevissimo, 

 convexo.postico elonyato mediocriter attenuato ; margine superiori et inferiori parallel)*, 

 subrectis ; striis concentricis magnis regularibus et crebris. 



Shell oblong, elongated, convex; umbones anterior, incurved, anterior side convex, 

 very short, its margin rounded ; posterior side elongated, superior and inferior borders 

 nearly parallel, horizontal, and slightly curved ; the lunule is excavated ; the sides of the 

 valves have regular, strongly impressed, and closely arranged longitudinal striations, which 

 nearly vanish as they approach the superior border. 



The form of this elegant species is intermediate between Ceromya concentrica and C. ex- 

 centrica, some examples approaching more nearly to the former, others to the latter shell, the 

 striations are strongly marked, rather larger than in C. concentrica, and there exists a 

 slight vertical depression upon the middle of the valves ; the umbones are rather depressed, 

 scarcely rising higher than the elongated superior border. 



Height, 15 lines; length, 22 lines; diameter through both the valves, 14 lines. 



Locality. Ponton, in the shelly beds ; also in the lower strata of Stamford, Morcot, &c. 



