Orthis. 



MOLLUSCA. 



1H» 



Lower Silurian Rocks at Hope, and near Barnstaple, Devon- 

 shire. 



24. Orthis multifurcatus. — The Many-forked Orthis, 

 pi. LIII. fig. 47. 



Orthis fabellum F var. Sowerby, Sil. Syst. p. 639, pi. 19, 

 fig. 8. 



Fan-shaped, moderately convex ; hinge line much shorter 

 than the width of the shell; base rounded; surface with a num- 

 ber of radiating, strong, forked ridges. Length eleven lines ; 

 width fourteen lines. 



Lower Silurian Rocks, east flank of the Caradoc. 



25. Orthis costata The Ribbed Orthis, pi. LIIL fig. 



50. 



Orthis ? costata. Sowerby, Sil. Syst. p. 639, pi. 21, fig. 11. 



Semicircular ; one valve a little conical ; hinge line slightly 

 arcuated ; umbones a little produced, with a large triangular 

 area beneath it ; sides somewhat angular ; surface with about 

 twenty sharp, radiating ribs. Length five lines and a half; 

 width seven lines and a half. 



Lower Silurian Limestone, Cefn, near Welshpool. 



26. Orthis Actoni^ Acton's Orthis, pi. LIIL fig. 38. 



Orthis ActonicF. Sowerby, Sil. Syst. p. 639, pi- 20, fig. 16. 

 Transversely obovate; one valve convex, the other flat; with 



about fourteen large radiating ribs, quadrifed or trifid at their 

 outer extremities. Length eight lines and a half; width eleven 

 lines. 



Closely resembling O. flahellum, but distinguished by its furcated ribs. 



Lower Silurian Rocks, Acton Scott, Church Stretton, and in 

 the Cambrian Rocks at Bala. 



27- Orthis semilunata The Semilunar Orthis, pi. 



LIII. fig. 51, 52. 



Orthis lunata. Sowerby, Sil. Syst. p. 603 and p. 61 1, pi. 5, 

 fig. 15. 



Transversely subquadrangular; finely and deeply striated lon- 

 gitudinally ; beaks not very prominent; hinge area small; lesser 

 valve with a slight, longitudinal, central depression ; internally 

 provided with two semicircular ridges. Length five lines ; 

 width seven lines. 



This species and orbiculata are so similar, that unless the specimens 

 are very perfect, it is nearly impossible to identify them ; the charac- 

 teristic internal ridges are the best criterion. 



Found plentifully in the Upper Ludlow Rock at Delbury ; 

 also in the lovvest beds of the Old Red Sandstone at Horeb 

 Chapel. 



28. Orthis expansa — The Expanded Orthis, pi. LIII. 

 fig. 26. 



Orthis expansa. Sowerby, Sil. Syst. p. 638, pi. 20, fig. 14. 



Semicircular, compressed ; hinge line parallel, internally 

 plaited near the margin ; muscular impression with deep fur- 

 rows; base slightly rounded. Length one inch and seven lines; 

 width two inches. 



Lower Silurian Rocks, Moel-y-Garth and Gaerfawr, near 

 Guilsficld. 



29. Orthis radians. — The Radiated Orthis, pi. LIII. 

 fig. 10. 



Orthis radians. Sowerby, Sil. Syst. p. 639, pi. 22, fig. 1 1. 



Semicylindrical, compressed; base concave; with about fif- 

 teen sharp plaits ; beaks produced. Length four lines and a 

 half; width half an inch. 



Lower Silurian Limestone, Goleugoed and Llandegley. 



30. Orthis callactis, p?—?\. LIIL fig. 22. 



Orthis callactis. Dalman, 1. c. p. 113. Sowerby, Sil. Syst. 

 p. 639. 



Nearly circular, but its width somewhat greater than its 

 length ; much compressed ; hinge line as long as the breadth ; 

 with about twenty radiating, slightly elevated ribs, which are 

 obsolete towards the beaks. Length seven lines ; width nine 

 lines. 



Resembles O.Jiabeltum, but the convex valve is considerably flatter. 



Lower Silurian Rocks at Cardoc Hills, Hope Mill, Shrop- 

 shire ; and Old Storridge Hill, Worcestershire. 



31. Orthis bilobata — The Two-lobed Orthis, pi. LIII. 

 fig. 39, 40. 



Orthis hilobata. Sowerby, Sil. Syst. p. 640, pi. 19, fig. 7. 



Transversely subquadrate ; hinge line parallel ; upper valve 

 concave, with a broad, subangulated, longitudinal, central eleva- 

 tion; lower valve convex, with a deep, broad, central, longitudi- 

 nal channel ; sides at the hinge line produced, and somewhat 

 rounded; surface with numerous, radiating ridges. Length one 

 inch and two lines ; width one inch and seven lines. 



Nearly allied to O. vespertilio, but more convex. 



Lower Silurian Rocks at Acton Scott, Horderley, and Upper 

 Cambrian Rocks, Bala. 



32. Orthis filosa. — The Threaded Orthis, pi. LIII. fig. 

 42. 



Orthis Jilosa. Sowerby, Sil. Syst. p. 630, pi. 13, fig. 12. 



Semioval, thin, much flattened ; hinge line somewhat longer 

 than the width of the shell; beaks hardly elevated; sides rather 

 straight; surface with numerous, thread-like, radiating stri*, 

 extending from the beaks to the margin. Length and width 

 ten lines. 



Wenlock Shale, Barrington and Oldcastle, Melvern. 



33. Orthis antiquata. — The Antiquated Orthis, pi. LIII. 

 fig. 24, 25. 



Orthis antiquata. Sowerby, Sil. Syst. p. 630, pi. 13, fig. 

 13. 



Semicircular, compressed ; hinge area narrow, as wide as the 

 shell; lesser valve flat; surface with numerous, radiating stri*, 

 a few larger ones, with many lesser ones intervening, parti- 

 cularly towards the margins, all of which are intersected by 

 somewhat remote lines of growth. Length six lines; width 

 eight lines. 



Wenlock Shale at Woolhope. 



34. Orthis hybrida The Hybrid Orthis, pi. LIII. fig. 



59,60,61. 



Orthis hybrida. Sowerby, Sil. Syst. p. 630 and p. 640, pi. 

 13, fig. 11. 



Nearly orbicular, a little wider than long; lenticular, but 

 thickest towards the beaks; valves equal; hinge line shorter 

 than the width of the valves; base rather straight; surface 

 with numerous, radiating stria?, which increase in number to- 

 wards the margins. Length five lines and a half; width six 



lines. 



The shortness of the hinge area of this species gives it the appearance 

 of an Atrypa, with which it forms a connecting link. 



Wenlock Shale at Hay Head, Walsall. 



35. Orthis lata The Wide Orthis, pi. LIIL fig. 29. 



Orthis lata. Sowerby, Sil. Syst. p. 640, pi. 22, fig. 10. 



