Orthis. 
MOLLUSCA. 
119 
Lower Silurian Rocks at Hope, and near Barnstaple, Devon¬ 
shire. 
24. Orthis multifurcatus _The Many-forked Orthis, 
pi. LIII. fig. 47. 
Orthis jiahellunri? 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. LIII. 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. 
Low’er Silurian Limestone, Cefn, near Welshpool. 
26. Orthis Actonia;. —Acton’s Orthis, pi. LIII. fig. 38. 
Orthis Actonuc. 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. Jlabellum, 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. 611, 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 lowest 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-Gartli and Gaerfawr, near 
Guilsfield. 
29* Orthis radians. —The Radiated Orthis, pi. LIII. 
fig. 10. 
Orthis radians. Sowerby, Sil. Syst. p. 639, pi* 22, fig. 11. 
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? —PI. LIII. 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. Jlabellum. 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 bilobata. 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. 
OrthisJilosa. Sow’erby, 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 stria*, 
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 stria?, 
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 striae, 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, vvitli which it forms a connecting link. 
Wenlock Shale at Hay Head, Walsall. 
35. Orthis lata _The Wide Orthis, pi. LIII. fig. 29. 
Orthis lata . Sowerby, Sil. Syst. p. 640, pi. 22, fig. 10. 
