BRITISH SILURIAN BRACHIOPODA. 119 



both Atrypa and Zygospira. In Atrypa the spiral coils are placed side by side, with their 

 apices directed towards the cavity of dorsal valve in a vertical manner, and its coils are 

 very numerous. In Glassia the coils, on the contrary, are few in number and are not 

 vertical, as in Atrypa, but laterally placed, so that their 

 extremities have their ends facing and touching each other 

 towards the centre of the shell. The character of the shell 

 itself also is in the two cases materially different. 



Glassia differs from Zygospira in the direction of its 

 spirals, the bases of the spiral coils in Zygospira being 

 somewhat as in Atrypa, and not directly facing the lateral 

 margins, as in Glassia. The loop also of Glassia, whilst 

 like that of Atrypa, differs from the loop of Zygospira. The Zygospira modesta, after Hall's 

 loop of the latter genus stretches right across the top of the ^' 



spiral cones from the primary lamellae on either side. Hall gave two figures of Z. 

 modesta. His fig. 1 is here copied for comparison ; but the loop in this figure, as proved 

 by Mr. Glass's researches, is misplaced. Hall's fig. 2 correctly represents the spiral 

 arrangement of Z. modesta, and corresponds with the figure I have given at p. 124 of 

 this Supplement. 



17. Glassia elongata, Dav. Sil. Sup., PI. VII, figs. 9, 10. 



Glassia elongata, Bav. Geol. Mag., New Series, vol. viii, p. 148, pi. v, figs. 3, 4, 

 April, 1881. 



Shell small, elongated oval, or obscurely pentagonal ; valves very gently convex, 

 straight, gently rounded or slightly indented in front, tapering posteriorly, broadest ante- 

 riorly ; beak small, incurved ; surface smooth. Spirals, &c., exactly as in Glassia obovata. 

 Length 4, width 3, depth 1^ hues. 



Obs. — While sorting a lot of specimens which Mr. Maw had obtained from extensive 

 washings of the upper part of the Upper Wenlock Shales (Tickwood beds) from below 

 the limestone, at the watercourse under Benthall Edge, Mr. Glass observed several speci- 

 mens of a small shell somewhat resembling Athyris Imviuscula, but broader in proportion 

 to length, and more flattened than in the last-named species. On experimenting upon 

 these, and after removing the shell and scraping the matrix, he was surprised to find 

 that the shell was provided with spirals arranged as in Glassia obovata, but usually with 

 only three convolutions. 



17 



