﻿Vol. 6 1.] SEQUENCE IN THE BRISTOL AREA. 289 



Productus semireticulatus (Martin), as interpreted in Davidson's 

 ' Monogr. Brit. Poss. Brach.' vol. ii (1858-63) Pal. Soc. pi. xliii, 

 %. 1. 



The characters of chief importance are : — The broad form ; the 

 sulcate or flattened medial area ; flattened wings ; strong semi- 

 reticulation. 



The adductors in the convex valve are attached more anteriorly 

 than is the case in the giganteid Producti (see below, p. 293). 



Evolution and mutation. — As already suggested, the typical 

 Productus semireticulatus was probably evolved from forms closely 

 allied to Pr. cf. Martini during the period represented by Z 2 . 



The typical form reaches its maximum at y and in the lowest 

 part of C. 1 Near the top of S lf the gens is represented by a muta- 

 tion with very long and numerous spines : this mutation has 

 given rise to the title of ' longisjoinus-bed ' for certain shales and 

 thin limestones which occur near the top of S x , in the Great 

 Quarry, Clifton, Avon section ; the shales are crowded with the 

 spines and tests of this mutation. With this spinous mutation, we 

 appear to have reached the end of a line of evolution, for no 

 examples of semireticulate Producti are again met with in the 

 Bristol or surrounding areas, until the occurrence of Pr. scabriculus 

 in Horizon e, at the very top of the Carboniferous-Limestone 

 Series. 



[Very common in the Mendip area, 1 and especially characteristic 

 of C, is a form similar to Productus semireticulatus var. concimius 

 (Sow.), as represented in Davidson's ' Monogr. Brit. Foss. 

 Brachiop.' vol. ii (1858-63) pi. xliii, figs. 9 & 10. This form may, 

 with great probability, be also traced back to Productus cf. Martini 

 and, apparently, represents a divergent line of evolution, charac- 

 terized by the gradual accession of a geniculate form and a coarser 

 type of ornament.] 



Productus 0. (PI. XXV, fig. 3.) 



General characters. — Hinge-line not exceeding the width of 

 the shell. Shell large, often very large. Test thick, so that the 

 pattern of the ribbing does not appear on the cast. 



Convex valve. — The body of the valve is almost globular, 

 but it contracts rapidly towards the beak; the flanks are steep. 

 The wings are flattened, and strongly marked off from the rest of 

 the valve. The ribbing is fine, but distinct; the spacing of the 

 ribs is maintained almost uniform over the entire valve, owing to 

 the intercalation of two or three fresh series of intermediate ribs. 

 All the individual ribs of each new series start, approximately, at 

 the same distance from the beak. The ribs of each new series are 



1 The evidence upon which these statements are founded rests, largely, upon 

 an examination of the specimens which Mr. T. F. Sibly, B.Sc, has so carefully 

 collected from the JBurrington Section (Mendips), in the course of preparing his 

 recently-published paper in the Proceedings of the Bristol Naturalists' Society. 



