10 MOLLUSCA FROM THE GREAT OOLITE. 



advanced stage of growth being very prominent, unequal, rather irregular, and commonly 

 covered with adherent shells ; some slight undulations or irregularities are visible upon 

 both descriptions of lines ; young individuals are more depressed and ovate, their lines are 

 very regular and distinct. It occurs not uncommonly in the planking beds throughout the 

 Minchinhampton district, but the surfaces of the valves are frequently much obscured by 

 adherent shells and adventitious matter entangled in the crossbarred surface. Height, 

 23 lines; lateral diameter, 21 lines. 



Localities. Minchiuhampton and Bisley Commons. 



PECTEN HEMICOSTATUS. Tab. I, fig. 16. 



? Var. of PECTEN VAGANS, Saw. 



Testa ovato-orbiculari incequivalvi subnequilaterd, valvd sinistrd convexd, lineis radi- 

 antibus irregularibus et lamellis concentricis crebris ornatd ; estate adulto costis radiantibus 

 magnis (51 distantibus plicis magnis instructis ; intervallis latis, costulis minoribus interdum 

 evanescentibus. Valvd deastrd depressd, lamellis concentricis crebris, interdum interrupts, 

 et sulcis radiantibus leviter impressis. Auriculis magnis intequalibus, costellatis. 



Shell inequivalve, nearly equilateral, ovately orbicular, the left valve convex, with 

 numerous irregular radiating lines crossed by closely arranged concentric lamellae ; the 

 adult condition has the valve more convex, with five large elevated plicated and distantly 

 arranged radiating costae ; the intervals are wide, each having a supplementary costa 

 more or less distinctly marked. The right valve is much more flattened, with concentric 

 densely arranged lamellae, sometimes interrupted, and a few radiating sulcations, which 

 are so faintly impressed, that they are only visible near to the lower border. Auricles 

 large, unequal, and costellated. 



The surface of the convex valve in progress of growth undergoes a striking change ; 

 in the young state it is beautifully reticulated, but has no indications of the costae which 

 afterwards distinguish it ; the adult shells have a form more convex, with five prominent 

 radiating costae, of which those at the sides are the smaller; the costae have a few large 

 irregular plications ; it is only shells of the largest size that have a supplementary costa in 

 each of the interstitial spaces. The imbricated lamellae of the right valve are more 

 prominent than those of Pecten vagans, and the sulcations are much less strongly 

 impressed, so that they can only be discovered by a close examination. It occurs not 

 unfrequently in the shelly beds of the formation ; but, from the general coarseness of 

 the deposit, the more delicate features of the surface are rarely preserved. The right valve 

 is delicate, and few specimens have been distinguished. 



Locality. The Minchinhampton district of the Great Oolite, throughout the 

 shelly beds. 



