PLATE XIV. 



SILURIAN SPECIES. 



Fig. 



1. Atrypa reticularis, Linn. Wenlock limestone, Rushall Canal. Specimen with one of its fringes 



almost entire. 



2. „ „ Impression of a specimen with marginal fringe almost complete. Wenlock 



limestone, Marines Bay. Mus. Geol. Survey, London. 



3. „ „ Small specimen with fringe. Wenlock shale, Pentland Hills. 



4. 4 a. „ „ From the Wenlock limestone, Dudley. 4 a, enlarged. 



5. ,, „ A specimen with fringe viewed in profile. Rushall Canal, Walsall. 



6 to 1 1 . „ A series of specimens of different ages and forms, from Wenlock limestone, 



Dudley. 



12. „ „ A variety, with small numerous ribs. Dudley. 



13. „ „ Ventral valve greatly enlarged, from a specimen in which the ribs are 



unusually angular, and often bifurcating. Wenlock shale; Mus. Geol. 

 Survey, London. 



14. „ „ (var. orbicularis, Sow.). Penkill Hill, Girvan, Ayrshire. 



15. „ „ Interior of dorsal valve (minus the spiral lamellae), to show the hinge-plate and 



muscular impressions. Wenlock limestone, Dudley. 



[A full account of the internal appendages, by Mr. R. P. Whitefield, 

 is given in the 19th Report on the New York State Cabinet, Dec, 1866.1] 



16. „ „ Interior of ventral valve. Dudley. 



17. „ „ A fine internal cast of dorsal valve. Wenlock Umestone, Usk. Museum 



of the Geological Society, London. 



18. 19. „ 18. Interior of ventral valve, from the Upper Llandovery, Huntley Hill. 



Mus. Geol. Survey, London. 19. Internal cast; same locality. 



20. „ „ Silicified cast, the spaces occupied by the spiral coils being left empty. 



Wenlock limestone, Malvern ; Mus. Geol. Survey. 



21, 22. „ „ Young shells with few ribs, approaching in character to A. aspera. 



Wenlock limestone, Dudley. 



' "The internal appendages of the genus Atrypa have been known to consist of a pair of spiral 



cones, placed side by side, with their apices directed towards the cavity of the dorsal valve By 



carefully cutting and preparing favorable specimens, I have found that in place of the short crural processes 



so often figured, there is an entire and continuous loop connecting the spiral cones, having its 



connection with the spiral ribbons at a point relatively much nearer to their origin on the hinge-plate : still 

 more distant, however, than the points figured by Mr. Davidson and others. This loop, so far as yet 

 observed, is confined to the rostral or posterior part of the shell, and never passes over or in front of the 

 spires, as in Prof. Hall's genm Zi/yospira. From its origin on the posterior portion of the first volutions 

 of the spires, the loop curves gently forward and upward ; the central or elevated portion lying between and 

 beliind the cones, and forming a more or less abrupt curve, or prolonged into a point directed towards the 

 dorsal valve," &c. 



