﻿Yol. 6 1.] SEQUENCE IN THE BRISTOL ABBA. 245 



BRACHIOPODS. 

 Athyrids. 



(i) Cliothyris. 

 Species-groups : — 



(1) Cliothyris Royssii. The typical form is abundant in M 



and K v 



A mutation (mut. (3) is abundant in K 2 ; it reaches a maximum at (3, 

 and is not uncommon in Z v 



(2) Cliothyris glabristria. Rare in K ; abundant through- 



out Z ; maximum in the lower part of Z 2 . 



(3) Cliothyris sp. (a transitional form between Cliothyris 



and Seminula). Top of Z Q and C ; maximum near y. 

 • ## 

 (ii) Seminula. 



Species-group: Seminula ficoidea. Rare in C; prolific 

 throughout S ; scarce in D. 



Spiriferids. 



(i) Reticularia. 



Species-group: Reticularia aff. lineata. Rare in K; 

 abundant in Z (maximum in Z x ) ; a mutation in C and S ; 

 a well-defined mutation in D 2 (Reticularia lineata). 



(ii) Spirifer. 

 Species-groups : — 



(1) Spirifer tornacensis, represented by Sp. aff. clathratus and 



its variety, and by Sp. aff. cinctus. Rare in K x ; in- 

 creasing in abundance in K 2 ; prolific in (3 and Z L ; 

 declining in Z 2 ; very rare after y. 



(2) Spirifer mosquensis, represented by a local variation. 



Rare in D 2 . 



(iii) Spiriferina. 



Species-group: Spiriferina octoplicata. Rare in K x ; 

 abundant in K 2 and at /3 ; scarce in Z r 



(iv) Syringothyris. 

 Species-groups : — 



(1) Syringothyris cuspiclata. 

 Mutational stages: — 



(a) Syringothyris aff. cuspidaia (cf. 8. subcuspidata) (probably 



more than one stage is here represented). Probably occurs 

 in M ; abundant from the top of K x to j3, and extends into Z. 



(b) Syringothyris cuspidata (the typical form). Occurs in Z x ; pro- 

 lific at the top of Z 2 and at y ; not uncommon throughout 0. 



(2) Syringothyris aff. laminosa. 



Common in Z , y, and C ; a mutation occurs in S x ; maxi- 

 mum in Z„. 

 Q.J.G.S. No. 242. ' s 



