374 MR. T. E. SIBLT ON THE CARBONIFEROUS [May I906, 



in the Upper Dibunojrfiylliim-Zone of the Mendips possess well- 

 developed area and tubuli, and exhibit the characteristic pitting of 

 the under-layer of the test. In their strongly-convex form and 

 very thick shell, however, they differ from the D x specimens and 

 resemble Daviesiella (Productus) llangollensis (Dav.) 'Monogr. Brit. 

 Foss. Brachiop/ (Palaeont. Soc.) vol. ii (1858-63) pi. xlv, figs. 1-6 & 

 pi. lv, fig. 9. 



Athyris. 



Athyris (Actinoconchtts) cf. lamellosa (L'Eveille). (PL XXXII, 

 figs. 1 a & 1 b.) 



The Mendip specimens, which arc all obtained from the Upper 

 Cleistopora-Zone, occur crushed in shale, so that their original 

 form is somewhat obscured. The mesial fold is slight, and starts 

 at some distance from the beak, while the mesial sinus is only 

 suggested on the anterior portion of the pedicle-valve. 



The concentric shell-expansions are excellently preserved in the 

 majority of specimens. They consist, in adult specimens, of about 

 16 strong, smooth lamella? which, when broken off at the base, 

 leave strong concentric ridges on the shell ; between these main 

 expansions, and especially well developed on the marginal portion 

 of the valves, are numerous similar but finer expansions, which, 

 when broken off, are represented by very fine concentric ridges. 



The figure in Davidson's ' Monogr. Brit. Foss. Brachiop.' (Paheont. 

 Soc.) vol. ii (1858-63) which most closely represents our form is 

 pi. Ii, fig. 14; but the specimen there depicted is less transverse 

 than our form, and further shows a marginal sinus in the brachial 

 valve, which none of our specimens possess. 



Athyris cf. glabristria (Phil.). 



= Cliothyris glabristria & mut.. Vaughan, Quart. Journ. Greol. Soc. vol. lxi 

 (1905) pp. 297, 298. 



Further collecting and much detailed work will be necessary 

 before the numerous mutational forms included in this compre- 

 hensive gens can be described with profit. For the present, the 

 several mutational forms are merely recorded in the faunal lists as 

 Athyris cf. glabristria, mut. Z x , mut. Z 2 , &c. 



In all our forms, the concentric shell-expansions consist of a 

 narrow, basal plate, which passes into a fringe. The width and 

 thickness of the basal plate vary considerably in different 

 specimens. 



Athyris cf. expansa (Phil.), Davidson. 



Compare Davidson, ' Monogr. Brit. Foss. Brachiop.' (Palaeont. Soc.) vul. ii 

 (1858-03) p. 82 & pi. xvi, figs. 14, 16, 17, pi. xvii, figs. 3, 3 a. 



Our specimens show considerable variation in form. The com- 

 monest form in the Mendip area is most closely represented by 



