B1VALVIA. 77 



Testa subellipticd, fornicatd, concentrice striatd, lineis radiantibusirregularibus, tenuis- 

 simis undulatis, scepissime obsoletis ; umbonibus parvis ; sulco elongate postico semper 

 notato. 



Shell subelliptical, ridged, concentrically and irregularly striated, with very fine 

 longitudinal waved lines, frequently obsolete ; umboues small ; posterior elongated, 

 external groove always visible. 



The few Great Oolite examples of this well-known shell are of much smaller dimensions 

 than those which are so abundant in the lower or Ammonitiferous beds of the Inferior 

 Oolite at Dundry, they are likewise more compressed ; they have only a moderate degree 

 of thickness, and the fine lines radiating from the umbones can rarely be discovered ; 

 these variations are such as might be expected to occur in a species which possesses so 

 considerable a geological range, and they are moreover precisely similar to those which 

 the species presents when it is found in the middle or freestone division of the Inferior 

 Oolite, in Gloucestershire. 



Dimensions of the larger Great Oolite specimens : 



Height, 14 lines; longitudinal diameter, 27 lines; diameter through both the valves, 

 8 lines. 



Localities. Minchinhampton Common ; Barnack, Northamptonshire ; Ponton, Lin- 

 colnshire. 



MYOCONCHA ACTION, D'Orbigny. Tab. Ill, fig. 17, 17. 



MYOCONCHA ACTION, D'Orbigny. Prodrome de Paleeont., p. 312, 1850. 



Testa ovato-oblongd subdepressd, marginibus superioribus et inferioribus parallelis, margine 

 postico subrecto, umbonibus parvis, depressis, plicis concentricis paucis irregularibus. 



Shell ovately oblong, the superior and inferior borders straight and nearly parallel, 

 the posterior border nearly square; the umbones very small and depressed, posterior 

 sulcus distinct ; concentric plications few and irregular. 



On comparison with MyoconcJia crassa, this shell is more depressed, less pointed at the 

 extremities, the posterior border more especially being quadrate ; the superior and inferior 

 borders are more nearly straight or parallel, and no radiating lines are visible upon the 

 surface. 



Length, one inch and three quarters ; height, one inch. 



Locality. Minchinhampton Common, where it occurs in the bed of coarse planking. 



MYOCONCHA ELONGATA. Tab. Ill, fig. 18. 



Testa soleniformi, clongatd, subdepressd, umbonibus parvis contiguis depressis, latere 

 antico angusto, postico latiore et compresso, marginibus superioribus et inferioribus rectis, 

 parallelis, plicis incrementi paucis tenuibus. 



Shell pod-shaped, elongated, rather depressed ; umbones small, contiguous, depressed ; 



