BIVALVIA. 71 



forwards, having a pit above it to receive the anterior lateral tooth of the other valve. Left 

 valve with two cardinal teeth, of which the anterior one is prominent, somewhat conical, 

 and disunited from the other, there is also a small approximate anterior lateral tooth. 

 Each valve has likewise a distant posterior lateral tooth, which is not very prominent. 



As SphoRra has a considerable general resemblance to Corbis : we will concisely indicate 

 the features whereby they are distinguished. In Corbis, the anterior side is the most 

 prominent ; Sphcera, is equilateral and oblique. The surface of Corbis is always cancellated 

 having a denticulated inner border ; Spheera, has its surface smooth, or is marked only 

 with the lines of growth, and the inner margin is acute and smooth. The hinge in the 

 right valve of Corbis consists of two narrow triangular teeth placed like the sides of the 

 letter V, the angle being at the umbo, the anterior lateral tooth being separate and 

 distinct ; in Sphcera, the cardinal teeth are thick, not angular or pointed, and the anterior 

 one forms a thickened oblique, lengthened process, before which there is no lateral tooth. 

 In the left valve the arrangement of the teeth is likewise different ; in Sp/tesra, the anterior 

 and larger cardinal tooth is obtusely conical and projecting ; in the other genus it is 

 trigonal and depressed, and the anterior lateral tooth is differently situated with respect to 

 the teeth of the other valve. Allowing, then, that SpAcera is nearly allied to Corbis, there 

 would appear to exist sufficiently distinctive characters to demand their separation sub- 

 generically. 



SPH.ERA MADRIDI. Tab. VII, fig. 14, 14a, b, c, d. 



CARDIUM MADRIDI, D'Archiae. Mem. Soc. Geol. Fr., torn, v, pi. 25, f. 7, 1843. 



CORBIS MADRIDI, If Orb. Prodome Paleont., i, p. 309. 



? CARDIUM INCERTUM, Phil. Geol. York., i, t. 11, f. 5, 1835. 



Testa crassd subglobosd, umbonibus magnis oUiquis et contiguis. Valms in atate 

 juniori Icevigatis subdepressis ; in estate adulto globoso plicis incrementi rugis, concentricis et 

 irregularibus. 



Shell thick, subglobose ; umbones large, directed obliquely forwards, and contiguous. 

 In the young state the valves are rather depressed and smooth ; in the adult state they 

 become much more globose, and acquire concentric, irregular, and prominent folds of 

 growth. 



Considerable variation occurs in the figure of this species, the more globose specimens 

 have the height and length of the valves almost equal, those which are more depressed 

 have a greater length laterally and are nearly smooth ; the latter characters are exhibited in 

 specimens from the Inferior Oolite of Leckhampton Hill, where it occurs somewhat rarely. 

 Our species ranks as one of the most abundant shells of the shelly beds of Great Oolite in 

 the Minchinhampton district, and we have also detected it at several positions higher in 

 the series, even to 120 feet above the Fuller's earth. 



Localities. Minchinhampton and Bisley Commons, in the Great Oolite ; in the shelly 

 roe stone of the Inferior Oolite of Leckhampton Hill ; and in the Forest Marble, near Frome. 



