CAKBONIFEROUS LIMESTONE BRACHIOPODA. 253 



By the great thickening of the margins, it is obvious that this 

 little shell is adult. It only approximates in the most remote 

 degree to one other Remit hyris that 1 know of, viz. one of the 

 varieties of H. acuminata, from which it is distinguished by its 

 very small size, and the length exceeding both the width and the 

 depth. 



Very rare in the carboniferous limestone of Derbyshire. 



[Coi, University of Cambridge.) 



Pentamerus carhonarius (M^Coy). 



Desc. Globose, more or less inequilateral ; hinge-line nearly as 

 wide as the shell, cardinal angles obtuse; lateral and greater 

 portion of front margin in one plane ; middle of front margin 

 abruptly raised into a very narrow, short, oblong or rounded 

 sinus ; commissure blunt from the meeting of the valves at a 

 very obtuse angle. Entering valve varying from semicircular 

 to rhombic, very gibbous ; beak large, prominent ; profile re- 

 gularly arched from the apex to its front margin, its greatest 

 depth about the middle of the length ; mesial ridge narrow, 

 flattened, prominent, and strongly defined from the front 

 margin to the apex of the beak, either simple, divided by one 

 mesial hollow, or divided into four narrow ridges near the 

 margin, each side with about seven very large, rugged, angu- 

 lar, irregular, subequal ridges, at six lines from the beak, 

 beyond which they either continue simple to the margin, 

 ^ or some or all of them dichotomize : surface rather rugged 

 and very coarsely granulo-punctate, or minutely pustular 

 under the lens, with strong thickened interruptions of growth 

 at the margin after nine or ten lines long. Receiving valve 

 extremely gibbous ; beak very large, usually slightly inclined 

 to one side, varying greatly in its inrollment, according to the 

 form of the cardinal area, which is sometimes nearly half as 

 high as wide, triangular, very slightly concave, and nearly at 

 right angles with the plane of the lateral margins, in which 

 case the beak is prominent, and only slightly incurved, the 

 greatest depth of the valve being a little in front of its apex, 

 and the profile arching very gradually from thence to the 

 front margin ; in other specimens the beak is inrolled so as 

 nearly to touch that of the entering valve, when the cardinal 

 area is greatly reduced, very concave and lying nearly in the 

 plane of the lateral margins, the profile being more than semi- 

 circularly curved ; mesial hollow very strongly defined by two 

 thick ridges from the apex to the narrow sinus in the front 

 margin, either simply hollowed, or bearing four ridges, much 

 smaller than the lateral ones ; lateral ridges about ten to 



