118 



MOLLUSCA. 



Orthis. 



12. Orthis plicata. — The Plicated Orthis, pi. LIL* 

 fig. 61. 



Orthis plicata. Sowerby, Geo. Trans. V. 2nd series, pi. 53, 

 fig. 10. 



Transversely elongated, depressed, with numerous sharp 

 plaits. Width twice its length. 



Found in the Devonian Shales at Barnstaple. 



13. Orthis tenuistbiata. — The Thin-striated Orthis, pi. 

 LIII. fig. 11, 12. 



Orthis tenuistriata. Sowerby, Geo. Trans. V. 2nd series, pi. 

 57, fig. 12. 



Semicircular ; base straight, with numerous, very fine striae ; 

 one valve convex, the other considerably flatter ; hinge area 

 narrow and parallel. 



Found in the Devonian Shales at Morebath. 



14. Orthis orbiculata The Orbicular Orthis, pi. LIII. 



fig. 6, 7, 8. 



Orthis orbicularis. Sowerby, Sil. Syst. p. 611, pi. 5, 

 fig. 16. 



Nearly orbicular, with fine, regular, deep, radiating stria;; the 

 larger valve very convex, with a slightly curved beak, furnished 

 internally with a straight, longitudinal ridge, with a curved one 

 on either side ; the other valve slightly convex, with a wide, 

 central depression ; hinge area very small. Length six lines ; 

 width seven lines. 



Abundant in the Upper Ludlow Rock at Ludlow; Delbury; 

 Sutton, near Wenloek ; Dog Hill, near Ledbury ; and Cwm- 

 nantgwyn, near Buleth. 



15. Orthis canalis. — The Canaled Orthis, pi. LIII. fig. 

 34, 35, 36. 



Orthis canalis. Sowerby, Sil. Syst. p. 640, pi. 20, fig. 8, and 

 pi. 13, fig. 12 a. 



Longitudinally semiovate ; one valve very convex, beak pro- 

 duced, pointed, and incurved ; the other rather flat, and with a 

 broad central furrow, a little convex along the sides; base some- 

 what pointed, and slightly depressed ; hinge line not so wide as 

 the valves ; surface with many fine, radiating striae, which are 

 more numerous towards the margins. Length and width six 

 lines and a half. 



Differs from O. eletiantula in its more elongated form, and in the beak 

 of the larger valve being longer and more incurved. 



Wenloek Shale, Tame Bridge, Woolhope, and Delves Green; 

 and also in the Cardoc Sandstone at Horderley and Whittings- 

 low, near the Cardoc, &c. 



16. Orthis testudinaria The Tortoise Orthis, pi. 



LIL* fig. 10. 



Orthis testudinaria. Dalman, p. 115, pi. 2, fig. 4. Sowerby, 

 Sil. Syst. p. 640, pi. 20, fig. 9. 



Nearly orbicular; with numerous granulated radii of unequal 

 length ; larger or upper valve very convex, being nearly sub- 

 conic, with an incurved beak ; the lower valve with a mesial 

 furrow ; hinge line almost as wide as the valves. Length six 

 lines and a half; width eight lines. 



This differs from O. elegantula in being more convex, and in the radia- 

 tions being stronger. It also differs in its internal structure. 



Found in the Lower Silurian Rocks at Gaerfawr, east and 

 south of Cardoc and Powis Castle. 



17. Orthis Lunata — The Moon-shaped Orthis, pi. LIIL 

 fig. 56, 57, 58. 



Orthis Pecten? Dalman, 1. c. p. 110, pi. 1, fig. 6. Hist. 

 Pet. Suec. p. 70, pi. 20, fig. 6. Sowerby, Sil. Syst. p. 638, pi. 

 21, fig. 9. 



Semioval, convex; with numerous, nearly equal, fine, slightly 

 raised striae, intercepted by almost equidistant lines of growth ; 

 hinge line almost as wide as the shell ; one valve nearly flat. 

 Length one inch and a half; width one inch and eleven lines. 



18. Orthis Pecten? — The Pecten-formed Orthis, pi. 

 LIIL* fig. 1. 



Orthis Pecten, var. Dalman, pi. 1, fig. 6. Hist. Pet. Suec. 

 pi. 20, fig. 6. Sowerby, Sil. Syst. p. 614. 



Transversely ovate ; depressed, with many sharp, radiating 

 sulci, with intermediate ones towards the margins. Length ten 

 lines and a half; width one inch and four lines. 



Sowerby says this is a much wider shell than the true O. Pecten. 



Found in the Aymestry Limestone, Aymestry and Coniston. 



19- Orthis rustica. — The Rude Orthis, pi. LIII. fig. 17. 



Orthis rustica. Sowerby, Sil. Syst. p. 624, pi. 12, fig. 9. 



Transversely subquadrate, depressed; hinge area rather large, 

 and triangular; surface uneven, with from forty to fifty rounded, 

 divergent ribs, and intermediate ones as they approach the 

 margins; base nearly straight. Length one inch and one line; 

 width one inch and a half. 



Wenloek Limestone, Wenloek and Valley of Woolhope. 



20. Orthis alternata — The Alternately-ribbed Orthis, 

 pi. LIII. fig. 18, 19. 



Orthis alternata. Sowerby, Sil. Syst. p. 624 and p. 638, pi. 

 19, fig. 6. 



Transversely obovately subquadrangular; with extremely nu- 

 merous, fine, unequal, raised striae, increasing in number towards 

 the margins ; hinge line shorter than the width of the shell. 

 Length ten lines; width thirteen lines. 



Lower Silurian Rocks, Whittingslow ; Cardoc, Soudley ; 

 Lower Lickey Ridge ; Bervvyns, &c. 



21. Orthis protensa — The Stretched-out Orthis, pi. 

 LIIL fig. 21. 



Orthis protensa. Sowerby, Sil. Syst. p. 638, pi. 22, fig. 8, 9. 



Semiovate, depressed ; hinge line nearly the width of the 

 valves ; unequally thick ; surface covered with sharp, linear, 

 divergent stria?, crossed by a few remote lines of growth. 

 Length eight lines ; width seven lines. 



Lower Silurian Rocks, Berwyns ; Goleugoed, and Meadow 

 Town, near Shelve. 



22. Orthis compressa — The Compressed Orthis, pi. 

 LIII. fig. 20. 



Orthis compressa. Sowerby, Sil. Syst. p. 38, pi. 22, fig. 12. 



Semiovate, lenticular, compressed; hinge line parallel; whole 

 surface covered with very numerous, close-set, punctated striae. 

 Length one inch ; width fourteen lines. 



Lower Silurian Rocks, Hope Quarry, near Shelve, Shrop- 

 shire. 



23. Orthis semicircularis. — The Semicircular Orthis, 

 pi. LIII. fig. 30, 31. 



Orthis semicircularis. Sowerby, Sil. Syst. p. 639, pi. 21, 



fig- 7. 



Semicircular, convex ; base slightly rounded ; surface covered 

 with about thirty sharp, radiating striae, increasing in number 

 towards the margin; umbo protruding. Length three lines and 

 a half; width half an inch. 



