372 SIR. T. F. SIELY ON THE CARBONIFEROUS [May I906, 



Proeectes concinnes (Sow.), mut. D 2 . (PL XXXII, figs. 3 a & 3 b.) 



Convex valve. — The rostral portion of the valve forms a more 

 or less rectangular, flattened, transverse area. The medial area is 

 flattened and broadly-sulcate. 



The sides are steep, but not square- cut, the shell spreading out 

 in a skirt-like extension. The ribs are fine on the rostral portion 

 of the valve, becoming broader on the ' skirt/ Semireticulation is 

 strongly marked on the wings, but somewhat indistinct over the 

 medial area. Spines are few and scattered on the main part of 

 the valve, but more numerous on the flanks. 



Discussion. — In this form, the convex valve forms a strongly- 

 arched vault over the flat portion of the concave valve; and a 

 section of the convex valve, taken through the hinge-line, is 

 transverse and more or less rectangular. 



Our form, which characterizes the Upper Dibunophyllum-Zone of 

 the Mendip area, agrees very closely with the type * of Productus 

 concinnus, figured by Sowerby in i Min. Conch/ vol. iv (1823) 

 pi. cccxviii, fig. 1 (left-hand figure only). 



Variation. — Variants, of the form here described and figured, 

 towards Productus costatus, J. Sow., are very common. These 

 are characterized by an irregular development of strong, broad 

 ribs on the anterior part of the shell, and by the conspicuous develop- 

 ment of rows of spines on the alate flanks. 



Proeectes 0. (PI. XXXII, fig. 2.) 



General characters. — Hinge-line usually less than, never 

 exceeding, the width of the shell. Shell transverse, large, often 

 very large. Test moderately thick. 



Convex valve. — The body of the valve is globose; longitudinal 

 convexity greater than transverse convexity. The beak is broad, 

 but narrows rapidly to its apex. The medial area slopes gradually 

 and imperceptibly into the wings, which are cylindrically rolled but 

 not produced. The wings are generally smooth, but occasionally 

 show very faint concentric wrinkles, which never extend far onto 

 the flanks. A row of short, curved spines projects from the hinge- 

 line. The ribbing is close-set and regular, and the spacing of the 

 ribs is maintained uniform by the intercalation of fairly regular 

 series of intermediate ribs. 



Resemblances and differences. — From its large size, broad 

 beak, and cylindrically-rolled wings, Productus <p might often be 

 recorded as Pr. giganteus. But Pr. giganteus, as represented in 

 Davidson's ' Monogr. Brit. Foss. Brachiop/ (Palseont. Soc.) vol. ii 

 (1858-63) pis. xxxvii-xxxviii, & pi. xxxix, figs. 1 &4, differs from 



1 I am greatly indebted to Dr. A. Vaughan, who (having recently examined 

 the type-specimen, preserved in the Natural History Museum) has placed the 

 results of his investigation at my disposal. 



