BIVALVIA. 23 



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

 oblique, elongated posteriorly, and produced into an auricle. Hinge with five large and 

 rather remote sulci; internal teeth two, large and anterior, margins of the valves 

 sinuated. 



This species forms a link connecting two very dissimilar groups of Gervillics ; it pos- 

 sesses the characteristics of the second or contorted group in a more modified form than 

 G, Hartmanni, Goldfuss, or G. tortuosa, the Gastrochana tortuosa of Phillips ; these two 

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

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

 very lengthened hinge fine ; the lines of growth are strongly marked upon the 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. Minchinhampton ; Normandy. 



Gervillia crassicosta. Tab. II, fig. 9. 



Testa valde ohliqud, elongatd, convexiusculd, auriculo antico rotundato, postico iruncato 

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

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



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

 short and emarginated ; radiating costse 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 this rare species we have only obtained three examples ; the hinge border is much 

 shorter than in Pteroperna cosiatula, the posterior wing being but little produced ; the whole 

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

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

 from P. costatula. 



Locality. Minchinhampton Common. 



Gervillia radians. Tab. VI, fig. 10. 



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

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

 subhorizontali, auriculis prominidis ; lateribus lineis radiantibus paucis obscuris ; aliis 

 concentricis dense dispositis. 



Shell large, very thick, inequivalve, very much contorted, the left valve very convex. 



