RHYNCHONELLA. 75 



the umbo ; the hinge margin of larger valve not much indenting the imperforated one. 

 The sinus begins to appear at a short distance from the beak, when it soon turns up 

 almost perpendicularly in the shape of a narrow tongue to meet the bent downward 

 extremity of the mesial fold. The sinus is divided longitudinally by a groove which be- 

 comes deeper as it approaches the extremity, the anterior portion of the sinus thus presenting 

 two convex rounded costse, much depressing the level of the lateral margin of the mesial 

 fold ; three or four small lateral plaits existing likewise on each side of the sinus in this 

 valve. Structure unpunctuated. Length 6, width 7, depth 8 lines. 



Obs. This species was discovered by M. Herault, engineer of the Mines, and he gave 

 to it the name of Ter. grimace, afterwards translated to the Latin name of Ter. ringens, by 

 Von Buch, to whom we are indebted for the figure and description of the shell. 



It is only lately, that this form has been noticed in England, where it was discovered 

 in the Inferior Oolite of Yeovil, Sherburn, Greenland, &c, in Gloucestershire by the 

 Officers of the Geological Survey, to whom I am indebted for the loan of the specimens 

 (figs. 13 and 14) illustrated in PI. XIV. On the Continent, the shell is found in the 

 Inferior Oolite of Moutiers, near Caen. 



In England, this species does not appear to have attained the dimensions of the French 

 shells, they are not as thick or rounded, seem more angular, and are, perhaps, more 

 properly speaking, a variety of the original type. Eig. 15 represents the Norman type. 

 Fig. 16 is given to show, that, although the great character of the species under consider- 

 ation is to have a single plait in the elevated mesial fold, still in some rare cases it may 

 become bidentated, and even tridentated, as may be seen in the remarkable specimen 

 kindly lent me by M. Deslongchamps ; a similar variation from the common type has been 

 noticed in R. cynocephala, and in other species. 



R. ringens is placed by M. D'Orbigny 1 in the Upper Lias (Terrain Toarcien), I 

 believe it to be, however, more properly located in the lower beds of the Inferior Oolite. 



68. Rhtnchonella subringens, Dav. Plate XIV, figs. 17, 17" 



be 



Diagnosis. Shell irregularly globular, as wide as long and deep ; beak acute, much 

 recurved, overlying, and almost touching the umbo, concealing the foramen and leaving 

 little space for the passage of the pedicle muscular fibres ; valves convex, gibbous, and 

 ornamented by seven large, strongly indented costse in the smaller, and six in the larger 

 valve, proceeding from the umbo and beak to the front and sides ; in smaller valve, a large 

 centra], more elevated plait forms a mesial fold, to which corresponds a deep sinus in the 

 other one. Structure imperforated ; length 4, width 4, depth 4 lines. 



Obs. This little shell has been obtained from the Inferior Oolite of Somersetshire by 

 the Officers of the Geological Survey. It seems to differ from R. ringens, with which it 



1 Prodrome, vol. i, p. 258. 



