from the Carboniferous Limestone. 425 



y^'q ; the three mid-ridges at margin occupy a space of 2 lines, 



six of the lateral ones occupy the same space, surface smooth. 



The great number of small obtuse dichotomizing plaits on the 

 sides distinguishes this from any of the varieties of A. flexistria, 

 A. ventilabrum, &c., as well as their extending quite to the beak, 

 and the disproportionally large mesial ridges separate it from the 

 A. bifera and A. cuboides, 1 have not seen the dental lamellae. 



Rare in the carboniferous limestone of Derbyshire. 



{Col. University of Cambridge.) 



Hemithijris lonff a {M^Coy). 

 Desc. Longitudinally trigonal, length exceeding the width or 

 depth, greatest width at one-third from the front margin, 

 lateral anterior margins obtusely rounded, front very obtusely 

 angulated in the middle ; posterior lateral margins long, nearly 

 straight, converging to the beak at an angle of 80° ; commis- 

 sure with a slight wave towards the entering valve in the pos- 

 terior half of the lateral margins ; anterior half of the lateral 

 margins nearly straight, with a very faint indication of one 

 plate on each side ; middle of the front margin elevated at an 

 angle of 85° with the plane of the lateral margins into a wide 

 tongue-shaped sinus with sigmoid sides, acutely angular in 

 the middle ; entering valve with the profile very slightly 

 arched, greatest depth about the middle of the length ; sides 

 convex, arched abruptly downwards on each side from the 

 obtuse mesial line ; receiving valve flattened or very slightly 

 convex for about three lines from the beak, after which the 

 very narrow sides alone are continued nearly straight to the 

 obtusely rounded anterior lateral angles, the wide mesial por- 

 tion being strongly depressed to fill the sinus in the front 

 margin ; beak rather large, very slightly incurved ; surface 

 smooth, witli a few obtuse imbrications of growth near the 

 margin ; tissue very coarsely fibrous, almost visible to the 

 naked eye ; dental lamellse in beak of receiving valve very 

 short, subparallel, slightly divaricating; entering valve very 

 minute. Length nearly 6 lines, proportional width ^^q, length 

 of entering valve ^Vo, depth of entering valve y^^, depth of 

 receiving valve y\j%, width of sinus y^^j^^, depth thereof y^^^^. 

 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 Hemithyris 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. 

 {CoL University of Cambridge.) 



