BRITISH SILURIAN BRACHIOPODA. 101 



specimens of J. vittata. All the specimens of A. spiriferoides which Mr. Glass and 

 myself have seen are either filled with a limestone matrix or with a dark bituminous 

 spar, which is sometimes quite black and entirely impervious to hght, and which even 

 when in a more favorable condition is very difficult to work with satisfactory results. In 

 this respect we have been very fortunate in our British Carboniferous Athyrh ; for the 

 specimens at Castleton, in Derbyshire, are filled with a spar beautifully transparent 

 and peculiarly favorable for working. Other slight modifications of the form of the loop 

 and attachments of Athyris, as represented by A.pla?io-sidcata, may still be discovered, but 

 as this species has been so fully and clearly worked out, the task of comparison with it 

 will be easy. 



At page 23 of my ' Monograph of British Oolitic Brachiopoda,' published in 1851, 

 I mentioned, while describing the spiral coils of Spiriferina, that the lamellae are always 

 thicker on the inner side of the circumference than on the outer, which tapers out into an 

 acute edge, and this character I find to prevail in all the spiral-bearing species that have 

 come under my notice. In some, however, the thickest part of the lamella is towards the 

 middle, I have also observed that spines often appear on the edge and the face of the lamella 

 fronting the sides of the shell. These spines, arising from an expanded base, are implanted 

 very irregularly, always directing themselves towards the exterior of the spire and in 

 general horizontally to it. The same character is seen on the loops of the Terebratulidse. 

 Spines have been detected by Mr. Glass under similar conditions on the spirals of A. 

 pluno-sulcata. 



The genus Athyris is represented in the Devonian, Carboniferous, Permian, and 

 Triassic formations, and now, resulting from the investigations of Mr. Glass, it is proved 

 also to exist in the Silurian formation. Experience has abundantly shown in our later 

 researches that until we are acquainted with the loops and connections of the spiral- 

 bearing species it is hazardous to determine to which genera or divisions any of them 

 should belong. Those species, therefore, that are classed with Athyris in my * Silurian 

 Monograph ' must be considered so provisionally, the only British Silurian Athyris with 

 which we are certainly acquainted at present being A. lavimcula. 



7. Athyris ljsviuscula, Sow., sp. Dav., Sil. Mon., PI. X, figs. 28 — 32 ; and Sil. Sup., 



PI. IV, figs. 24 to 26. 



Terebratula l^viuscula, Sow. Sil. Syst, pi. xiii, fig. 14, 1839. 

 Meristella nitida, Dav. Sil. Mon., p. 114 (not Atrypa nitida, Hall), 1867. 



Marginally of a somewhat elongated, pentagonal shape, truncated, and slightly 

 indented in front, broadest about the middle, tapering posteriorly. Valves almost equally 

 convex, with a slight median depression near the front in the ventral valve. Beak strongly 



