﻿Vol. 64.] CARBONIFEKOUS LIMESTONE OF THE MIDLAND AREA. 77 



Conch.' vol. iv, 1823, pi. eccxviii, figs. 2-6) occurs commonly in D.. 

 in certain localities, and less frequently in D3. 



A small, very globose shell, identical with that figured by 

 Davidson, 'Monogr. Brit. Poss. Brachiop.' (Palseont. Soc.) vol. ii 

 (1858-63) pi. XXXV, figs. 19 & 19 a, is of rare occurrence in D,, 

 and Dg. 



The specimens which I refer to Productus setosus, Phil., exhibit 

 the following distinctive characters, compared with Pr. longispinus. 

 The shell usually is considerably larger, and the ribbing is much 

 coarser : in the more typical specimens, the ribs increase very 

 little by bifurcation : the convex valve exhibits a sharp, mesial, 

 frontal elevation, which is generally ornamented like the remainder 

 of the shell. [Compare Phillips, ' Geol. Yorks.' vol. ii (1 836) pi. viii, 

 fig. 17 ; & Davidson, ' Monogr. Brit. Foss. Brachiop. (Palseont. Soc.) 

 vol. ii (1858-63) pi. xxxv, fig. 16.] 



Productus setosus, Phil., var. tissingtonensis, nov. (PI. I, 

 figs. 6 a & 6 6.) 



Description. — The form approximates to that of a typical 

 Productus longispinus^ but the wings (which are rarely preserved) 

 are somewhat more extended. 



Convex valve. — The flattened, rostral portion of the valve is 

 covered with coarse, rounded, radial ribs, gradually increasing in 

 thickness towards the margin ; semireticulation is strong and 

 regular. The flank-ribs show a fairly-regular increase in width 

 towards the margin, with a little irregular thickening. On the 

 frontal portion of the valve, the ribs show a highly-irregular 

 behaviour (PI. I, fig. 6 ?>). Some remain of practically uniform 

 thickness ; others die out altogether ; while others again become 

 greatly broadened and often bifurcate. A sharp mesial fold, 

 originating in the fusion of two or more ribs, starts some distance 

 from the margin, and rapidly increases in prominence towards the 

 margin : it may be either smooth or faintly ribbed. Two specially- 

 strong ribs are frequently developed, bordering the flanks, one on 

 each side of the mesial fold. One or two spines project from each 

 wing, and a few strong spines are developed on the flanks and on 

 the frontal portion of the valve. Two specially-prominent spines^ 

 are frequently present, one on each of the thickened ribs bordering 

 the flanks. 



Discussion. — Productus setosus var. tissinytonensis is com- 

 pletely distinguished from the typical form of Pr. setosus by the 

 remarkable irregular nature of its ornament. The variety tissing- 

 tonensis occurs in D3, and is especially common in that subzone at 

 Tissington, near Ashbourne. In virtue of its external ornament, 

 this variety may be regarded as indicating the convergence of 

 Productus setosus with the more highly-ornate forms of Pr. costatus, 

 Sow., the latter also being especially characteristic of D . 



