﻿Vol. 6 1.] 



SEQUENCE IN THE BRISTOL AREA. 



295 



Chonetes aff. papilionacea denotes the group of early papiliona- 

 ceans ; it comprises large forms, in which one valve is 

 markedly convex, and the pits are elongated and well-spaced. 



It seems probable that Chonetes Dalmaniana, de Kon. (Davidson, 

 ojo. cit. pi. xlvi, fig. 7), is the link between the Tournaisian forms 

 of Ch. cf. hardrensis and the present group : for, where this form 

 is abundant, Ch. cf. hardrensis becomes rare, and the convex early 

 papilionaceans make their first appearance. 



In some localities the convexity of the early papilionaceans is so 

 great that the shells may easily be mistaken, at first sight, for 

 Productus giganteus. This form I have denoted by Chonetes cf. 

 comoides, as I am unacquainted with the internal characters. 



Chonetes aff. comoides (J. Sow.). (PI. XXVI, fig. 4 & text-fig. 4.) 



Compare Davidson, ' Monogr. Brit. Foss. Brachiop.' vol. ii (1858-63) pi. Iv, 

 figs. 6-8. 



This is a thick-shelled papilionacean of 



Fig. 4. — Interior of a convex valve of Chonetes aff. 

 comoides. (The teeth are omitted, and the 

 area is ivorn down, so that it shows the tubidi, 

 t, which communicate with the bases of the. 

 spines.) 



a = Adductors. 



c ='Auxiliary adductors. 



s = Medial septum. 



r = Diductors. 

 L = Depressions. 



considerable con- 

 vexity, charac- 

 teristic, so far 

 as I know, only 

 of the Dihuno- 

 phyllum - Zone 

 and the Upper 

 Seminula-Zone, 

 where it is 

 associated with 

 Productus gi- 

 ganteus. 



In area, teeth, 

 and the row of 

 spines along the 

 cardinal ridge, 

 it is a typical 

 Chonetes, but 

 the muscular 

 impressions are 

 practically iden- 

 tical with those 

 oiPr. giganteus. 

 From Ch. co- 

 moides our form 

 differs only in 

 less convexity 

 at the beak, and 



injthe presence of distinct tubuli, which run obliquely through the 

 shell, under the area, and communicate with the bases of the spines 

 on the cardinal ridge. It is a remarkable instance of the approxi- 

 mation of characters in cognate forms, existing at the same time, 

 that, while Chonetes aff. comoides simulates Productus giganteus in 

 all its characters (except area and teeth), it is not unusual to find 



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