BIVALVIA. 23 



valve flat, sometimes concave, its umbo terminal, depressed, and acute ; binge line very 

 obbque, elongated posteriorly, and produced into an auricle. Hinge witb five large and 

 ratber remote sulci; internal teetb two, large and anterior, margins of tbe valves 

 sinuated. 



Tbis species forms a link connecting two very dissimilar groups of Gervillia ; it pos- 

 sesses tbe characteristics of tbe second or contorted group in a more modified form tban 

 G. Hartmanni, Goldfuss, or G. tortuosa, tbe Gastrochama tortuosa of Phillips ; tbese two 

 latter species pertain to tbe Inferior Oobte, and tbe present one to tbe Great Oolite. 

 The degree of contortion varies considerably in individuals ; tbe young specimens have a 

 very lengthened hinge line ; the lines of growth are strongly marked upon tbe left valve, 

 and there are two short obscure ribs diverging from the umbo, these, however, disappeared 

 in the progress of growth, and the posterior extremity became more produced. 



The right valve is more delicate, and is found more rarely than the other ; the same 

 circumstance occurs likewise in Normandy, where M. Deslongchamps had not seen the 

 right valve when he described this species. It ranks as one of the most abundant and 

 characteristic bivalves of the Great Oolite ; it occurs indifferently in all the shelly beds. 

 Localities. Mincbinhampton ; Normandy. 



Gervillia crassicosta. Tab. II, fig. 9. 



Testa valde obliqud, elongatd, convexiusculd, auricido antico rotundato, postico truncato 

 et brevi ; costis radiantibus subundatis, elatis majoribus 8, alternatim minoribus, et cum 

 striis transversis numerosis, indentatis, latero postico elongato .- valvd dextrd ignotd. 



Shell very obbque, elongated, convex, anterior auricle rounded, posterior auricle 

 short and emarginated ; radiating costaa slightly waved, elevated, the larger 8 in number, 

 distant, and alternating with as many smaller, and impressed with numerous rather 

 indistinct transverse striae : posterior and inferior extremity elongated and slightly 

 acuminated : right valve unknown. 



Of tbis rare species we have only obtained three examples ; the hinge border is much 

 shorter than in Pteropema costatula, the posterior wing being but little produced ; the whole 

 contour of the shell is very oblique, and the larger costse are very prominent ; the greater 

 degree of obliquity, convexity, and alternation of the costae, readily serve to distinguish it 

 from P. costatula. 



Locality. Mincbinhampton Common. 



Gervillia radians. Tab. VI, fig. 10. 



Testa magna crassd inaquivalvi valde contortd, valvd sinistra arcuatd, umbone magno 

 incurvo, valvd dextrd oblique-concavo, umbone depresso ; margine cardinali elongato 

 subhorizontali, auriculis prominulis ; lateribus lineis radiantibus j)aucis obscuris ; aliis 

 concentricis dense dispositis. 



Shell large, very thick, inequivalve, very much contorted, tbe left valve very convex, 



