﻿Vol. 6 1.] SEQUENCE IN THE BRISTOL AREA. 29^ 



glabristria (fig. 5) and by Davidson's figures of 'Athyris' ambigua 

 (' Monogr. Brit. Foss. Brachiop.' vol. ii, 1858-63, pi. xvii, figs. VS 

 & 14). 



The absence of fringes (and therefore of external striations), to- 

 gether with the more Terebratuloid form, distinguishes Seminula 

 from Cliothyris. The presence of internal spires separates Seminula 

 from Dielasma. 



Seminula eicoidea, Vaughan. 



Described and figured in Proc. Bristol Nat. Soc. n. s. vol. x (1903) pp. 122-24 

 & pi. ii, figs. 1-1 b. 



Seminula ambigua (Sow.), Davidson. 



As represented in Davidson's ' Monogr. Brit. Foss. Brachiop.' vol. ii 

 (1858-63) pi. xv, figs. 15-18 & pi. xvii, fig. 11 (Athyris ambigua). 



I have seen very few specimens of this species in the Bristol 

 area ; the most typical of these were found in the Failand district 

 at Horizon d. Mr. T. F. Sibly has, however, quite recently found 

 the species abundant at the top of the Syringothyris-Zone, in the 

 Burrington section (Northern Mendips), and near Weston ; it is 

 by his kindness that I am permitted to make this important 

 statement, as to the zonal position of this well-known form. 



A well-marked mutation occurs at the top of S 2 and in D. 



Reticularia. 



Reticularia aff. line at a (Martin). 



The Bristol form, from the Zaphrentis-Zone, has usually a much 

 deeper sinus than is represented in Davidson's ' Monogr. Brit. Foss. 

 Brachiop.' vol. ii (1858-63) pi. xiii, fig. 9 & pi. xi, fig. 10 

 (* Spirifera ' lineata), which are, however, the two figures that 

 most closely represent our form. 



This species has commonly been referred to ' Spirifer glaberj 

 but the close-set radial ribs, under the outer layer of the test, and 

 the internal characters of septa and muscular scars, leave no doubt 

 as to its true generic position. 1 The typical lineation is usually con- 

 spicuous, but the surface is occasionally almost smooth. Impressions 

 of the external surface often exhibit the concentric rows of twin- 

 pores very beautifully. 



The assimilation of this species to Cliothyris glabristria in general 

 form has already been pointed out, but the well-developed area in 

 Reticularia aff. lineata completely distinguishes the two forms. 



' Spirifer glaber ' (Martin), Davidson. 



This species, as interpreted by Davidson, stands in need of careful 

 revision, based on differences of internal structure. The typical 

 Martinia glabra has not yet been met with in the Bristol area. 



1 Of. James Hall, ' Thirteenth Ann. Rep. of the State Geologist N. Y. 

 1893-94) : Introd. to the Study of Brachiop.' pt, ii, pi. xxv, fig. 12. 



