﻿Vol. 6 I.] SEQUENCE ITS THE BRISTOL A.KEA. 303 



Beak small, narrow, pointed, curved, but not recurved. Area 

 and delthyrium usually exposed to view. 



Internal characters. — Pedicle- valve: Cardinal area repre- 

 sented by two narrow, concave strips, sharply marked off by ridges. 

 Dental plates well developed, and forming the walls of the pedicle- 

 cavity. No spondylium. Brachial valve (see fig. 6, p. 302): 

 Hinge-plate formed of two concave plates, which are separated 

 by a gap at the beak, but are united below. Mesial septum well 

 developed, and split near the beak into two separate sheets, which 

 form the cruralium and support the hinge-plates. Dental sockets 

 crenulated. 



Dimensions. 



Average transverse dimension = 115 millimetres ; extreme = 14 mm. 

 Average axial dimension = 10 mm. ; extreme = 12 mm. 

 Average normal dimension = 7 mm. 



Trapezoidal intersection of fold and sinus : depth = 5 - 25 mm, ; width, 

 top and bottom = 275 mm. and 6 mm. 



This shell has previously been recorded as a variety of Rhynchonella 

 pleurodon, but it is so easily distinguished from typical examples 

 of that species that mere convenience suggests its separation under 

 a distinct specific name. 1 This course is rendered obligatory by 

 the fact that Camarotoecliia mitcheldeanensis occurs in vast profusion 

 in the Bristol and surrounding areas, and always characterizes the 

 Lower Tournaisian rocks. 



C. mitcheldeanensis differs from the typical Rliynclionella pleurodon 

 (such as is represented in Davidson, ' Monogr. Brit. Foss. Brachiop.' 

 vol. ii, 1858-63, pi. xxiii, fig. 4) in the following characters : — 



(1) The shell is much smaller ; (2) the fold is not relatively broad, and 

 the intersection of the fold and sinus is not rectangular, but markedly 

 trapezoidal ; (3) the median line is strongly convex under the beak 

 and becomes, continuously, less convex towards the front, but is 

 never absolutely flat. 



Rhynchonella triplex, M'Coy, differs from our species, in the fact 

 that the ribs in Rli. triplex become indistinct under the beak and 

 the flank-ribs are limited to only three prominent ones. 



Camarotoecliia, -pleurodon (Phil.), as represented by the figures in 

 Davidson, op. cit. pi. xxiii, includes forms from several horizons 

 (Z., at Clevedon, D 2 at Wrington). 



EXPLANATION OP PLATES XXII-XXIX. 

 Plate XXII. 

 Figs. 1 & la. Syringopora 9. 



Fig. 1. Exterior ; natural size. 



Z 2 -subzone ; Clevedon (Somerset). 

 1 a. Horizontal section ; X 1*25. 



Z 2 subzone ; Clevedon (Somerset). 

 Figs. 2-2 e. Zaphrentis aif. Phillipsi, Edwards & Haime. 

 Fig. 2. Exterior ; natural size. 



Zaphrentis-Zone ; Avon section, Clifton (Gloucestershire). 

 (Collection of J. W. Tutcher, Esq.) 



1 The specimens figured in PI. XXVI had been labelled ' var. or n. sp. 



