BIVALVIA. 105 



The extreme tenuity of the test will account for its uniformly bad state of preservation 

 and rareness. It occurs in a bed of soft shelly Oolite, which is situated about the middle 

 of the shelly beds, and abounds with valves of Tancredia. 



Length, 2j inches; height, If inches; diameter through both the valves, 7 lines. 



Locality. Minchinhampton Common. 



GRESSLYA PEREGRINA, var. ROSTRATA. Tab. X, fig. 7. 



GRESSLYA ROSTRATA, Agasziz. Etud. Crit., t. 126, fig. 7, 8. 



Testa ovato cuneiformi, antice rotundatd, postice elongatd et acuminatd, basi subrectd. 



Shell ovate or somewhat cuneiform, rounded anteriorly, produced and pointed 

 posteriorly ; basal margin nearly straight. 



The posterior side is somewhat compressed, forming an angle which extends obliquely 

 from the umbones to the infero-posterior extremity, and there forms a pointed termination. 



Height, 13 lines ; lateral diameter, 19 lines ; diameter through both the valves, 10 lines. 



Locality, The southern side of Minchinhampton common, where small openings in 

 the Stonesfield slate have afforded a few of the internal moulds. The genus never occurs 

 in the shelly beds of the formation. Marls of the Ostrea acuminata (fuller's earth). 



CEROMYA, Ag. 



Shell corcliform or oval, very inequilateral, ventricose ; umbones large, contiguous, 

 incurved, involute ; lunule excavated ; anterior side convex, its border rounded ; posterior 

 side elongated and more flattened, its border either closed or having a slight aperture ; 

 ligament narrow, external. The surface is ornamented with one or more series of ridges 

 and sulcations, which are longitudinal but not always concentric. In certain species a 

 change in the direction of the ridges occurred at a certain period of the growth ; substance 

 of the test thin, almost papyraceous. Hinge edentulous ; a lengthened lamina beneath the 

 ligament in the left valve is received into a groove beneath the lamina of the opposite 

 valve ; there is also in the right valve an obliquely elongated posterior rib or internal 

 depression, which, unlike that of Gresslya, is visible upon the surface of the test ; muscular 

 and pallial impressions rarely distinguishable ; the anterior impression is pyriform, elongated 

 upwards, and jagged or fringed irregularly, as in Pholadomya and Gresslya. 



The variety of figure in Ceromya is very considerable ; Ceromya similis, Lye., in its 

 elongated and compressed form approaching to that of Gresslya ; the opposite figure is 

 exemplified by C. Bajociana, D'Orbigny, which has the short ventricose aspect of hocardia, 

 between these there is every gradation of figure. Ceromya occurs rarely in the shelly 

 beds of the Great Oolite, the valves being most commonly disunited, the tests are then 

 preserved ; in other situations without shelly detritus the valves are united, but the tests 

 have disappeared. 



14 



