﻿Yol. 6 1.] SEQUENCE IN THE BEISTOL AREA. 291 



characters are exhibited, in the same specimen, only by the most 

 typical members of the group : — 



Form elongate, narrowing uniformly to the beak, with tall and steep flanks. 

 Beak narrow and strongly arched. Hinge-line short, with short wings or ears. 

 Ribs fine and sharp, with very narrow intermediates. Concentric wrinkles 

 indistinct on the medial area, but becoming deep pleats on the wings. Test 

 thin, so that the cast takes the pattern of the ribbing. 



Mutational Forms. 

 Mutation C. 



Wings usually flattened, with very strong pleats. The ribbing 

 on the flanks is comparable in sharpness and alternation with that 

 of an Orthotetes. The form is markedly broad, anteriorly. 



This form is very rare in Z v l more common in Z 2 , and abundant 

 in C. It has already been suggested that Productus 0, which 

 characterizes S„ is perhaps merely an aberrant variant of the gens 

 of Productus Cora [Dav.]. 



Mutation S a . (PL XXV, fig. 4.) 



A small form, not specially widened anteriorly. The short,, 

 usually-convex ears are strongly wrinkled, but not actually pleated. 

 The ribbing is fine and of the more normal type, as described under 

 Productus 0. Many specimens show broad, irregular wrinkles over 

 the entire valve. 



This form is especially characteristic of S„. 



PI. XXV, figs. 4a & 4b illustrate a closely-related form, occur- 

 ring in S 2 and D 1? which is remarkable for its extremely-fine 

 ribbing. 



Differences. — Productus Cora [Dav.] is likely to be confused 

 only with Pr. hemispJiericus ; the differences between the two 

 forms are pointed out in the remarks on the latter species. 



Evolution. — The evolution of the gens of Productus Cora 

 [Dav.] appears to have been carried out in a distant region, and the 

 early forms found in the Bristol area were probably introduced 

 by migration. This suggestion is rendered probable by (1) the 

 extreme rarity of the earliest members, and (2) the absence in 

 our area of transitional forms, linking them with pre-existing 

 Producti. 



Peoductus hemispheeicus, J". Sow. (PL XXV, fig. 5.) 



One of the original types is figured in Davidson's ' Monogr. Brit. 

 Foss. Brachiop.' vol. ii (1858-63) pi. xl, figs. 9-9 b. 



The shell is usually transverse, and the hinge-line does not exceed 

 the width of the shell. The beak is convex, but not strongly arched. 

 The medial area slopes gently into the flanks. The wings are short, 

 and somewhat cylindrically rolled. The ribbing is of the type 

 already described under Productus 6. Concentric wrinkles are, 

 as a rule, well marked on the wings, but are indistinct across the 



1 The only specimen that I have seen from Z l appears to be easily 

 distinguishable. 



