120 



MOLLUSCA. 



Orthis. 



Transversely semicylindrical, depressed ; base somewhat 

 straight ; sides rounded ; surface with Hnear, unequally thick, 

 divergent striae. Length five lines ; breadth nine lines. 



Nearly allied to Lcptiena lata, but differs in both valves being convex, 

 in being destitute of spines on the hinge, and in the unequally sized striae. 



Lower Silurian Rocks, Gorllwyn ; Goleugoed and Berwyns, 

 Caermarthenshire. 



36. Orthis alata The Winged Orthis, pi. LIIL fig. 



13, 14. 



Spirifer alatus. Sowerby, Sil. Syst. p. 638, pi. 22, fig. 7, 

 upper figures. 



Semicircular; hinge line parallel, with extended, cuspidate, 

 auriform processes, slightly inflated in the middle of the valves; 

 surface with about eighteen acute, radiating plaits, larger in the 

 centre of the valves, and smaller and closer towards their upper 

 portions; sides slightly crenated. Length five lines and a half; 

 width nine lines and a half. 



Lower Silurian Rocks, Mount Pleasant and Pensarn, Caer- 

 marthen. 



37. Orthis pennatus. — The Unequal-winged Orthis, pi. 

 LIIL fig. 4, 5. 



Spirifer alatus. Sowerby, Sil. Syst. p. 638, pi. 22, fig. 7, 

 lower figures. 



Longitudinally semiovate, oblique, inflated; hinge line paral- 

 lel, extended on either side into long, auriform processes, one 

 of which is considerably longer and more acute than the other ; 

 surface with numerous, acute, arcuated, radiating ribs, which 

 are smaller and more numerous towards the margins, with two 

 or three remote, slightly marked lines of growth ; margins 

 slightly crenated. Length six hnes ; width seven lines. 



Lower Silurian Rocks, Mount Pleasant, Caermarthen. 



38. Orthis interliniata. — The Interlinear Orthis, pi. 

 LIIL fig. 1, 2, 3. 



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

 53, fig. 1 1, and pi. 54, fig. 14. Phillips, Pal. Fos. p. 63, pi. 26, 

 fig. 106. 



Transversely elliptical, much compressed ; larger valve not 

 very convex; beak a little prominent; smaller valve with a 

 shallow depression ; hinge line little more than half the width 

 of the valves ; base nearly straight ; surface with numerous, 

 fine, filiform, radiating, unequally long striae, which are stronger 

 towards the margins of the valves; convex valve with nearly 

 equal muscular impressions, and the subrostral plaits arcuated 

 so as to include a cordiform area ; impressions of the cardinal 

 teeth in the other valve broad. 



Sowerby remarks as to this species, that it fonns a link serving to 

 connect five other species. It approaches O. lata in its wide form, and 

 irregularity of its strise; it resembles O. orhicularis and O. canalis in the 

 depression of the cardinal teeth of the flatter valves, and these species are 

 exact analogues of O. lunata and O. testudinaria. 



Abundant in the Devonian Shales at Petherwin, Croyde, and 

 Leary, North Devon ; Barnstaple, Morebath, and Landlake, 

 Cornwall. 



39. Orthis parallela. — The Parallel Orthis, pi. LIIL 

 fig. 9 and 16. 



Orthis parallela. Phillips, Pal. Fos. p. 64, pi. 26, fig. 109> 

 a, h, c, d. 



Oblong-oval, considerably compressed ; hinge line narrow, 

 occupying about half the width of the valves; base wide; deeper 

 valve with a slightly tumid, mesial line ; flatter valve with a 



hollowed mesial depression ; -surface with numerous, sharp, 

 unequally long and short, radiating striae, crossed by lines of 

 growth ; subrostral ridges of the deeper valve parallel, and 

 much lengthened, and including a long, divided area. 



Devonian Shales, Pilton and Brushford, North Devon ; and 

 Petherwin, Cornwall. 



40. Orthis granulosa. — The Granular Orthis, pi. LIIL 

 fig. 15. 



Orthis granulosa. Phillips, Pal. Fos. p. 65, pi. 26, fig. 111. 



Nearly orbicular, considerably compressed, and lenticular ; 

 hinge line somewhat more than half the width of the valves ; 

 surface with numerous, fine, granulated, unequally long striae, 

 increasing to double the number towards the margin, and pro- 

 ducing a bordered aspect. 



Devonian Shales at Hope, near Torquay, South Devon. 



41. Orthis arcuata The Arquated Orthis, pi. LIIL 



fig. 23. 



Orthis arcuata. Phillips, Pal. Fos. p. 64, pi. 26, fig. 107. 



Transversely oblong-ovate, very much compressed ; one 

 valve uniformly convex, the other with a slight mesial furrow ; 

 whole surface with very fine, divergent striae, arcuated towards 

 the sides. 



This species is subject to variety : var. a with the sulcated valve flat ; 

 var. 6, the sulcated valve convex on both sides of the mesial furrow. 



Distinguished from O. interliniata by its more lengthened shape, and 

 greatly arcuated and very fine striae. 



Devonian Shales, Hope, near Torquay. 



42. Orthis longisulcata. — The Long-furrowed Orthis, 

 pi. LIIL fig. 37. 



Orthis longisulcata. Phillips, Pal. Fos. p. 62, pi. 26, fig. 

 105. 



Transversely elliptical, compressed ; sides rounded ; hinge 

 line a little arcuated ; surface with very numerous, fine, divari- 

 cating striae, which are interrupted by concentric, imbricated, 

 distant lines of growth ; internal plates divergent, situate near 

 the beak of the lower valve. 



Devonian Shales, Watersmeet ; Woodabay ; West Lee, and 

 Linton, North Devon. 



43. Orthis concentrica. — The Concentric Orthis, pi. 

 LIIL fig. 41. 



Orthis concentrica. Portlock, Geo. Rep. p. 452, pi. 37, 

 fig. 1. 



Semicircular, compressed ; hinge line nearly parallel ; mus- 

 cular impressions deeply furrowed, internally plaited near the 

 margins ; surface with fine, divergent, filiform striae. 



Silurian Strata, Tyrone, Ireland. 



44. Orthis umbraculum. — The Shaded Orthis, pi. LIIL 

 fig. 32, 33. 



Orthis umbraculum. Portlock, Geo. Rep. p. 456, pi. 37, 

 fig. 5. 



Almost semicircular ; hinge line nearly parallel ; lower valve 

 slightly convex towards the beak, but nearly flat at the margin ; 

 cardinal area low, and strongly sulcated ; upper valve flat, with 

 a low, triangular, cardinal area; surface with twelve or thirteen 

 filiform costae, emanating from the umbones, and radiating 

 towards the margins, with very numerous, extremely fine, 

 intervening stria?, the dichotomy being by insertion, and not by 

 furcation ; besides the striae, by the aid of a lens, extremely 

 fine, longitudinal lines are observable. 



Shale of the Silurian series, Fermanagh, Ireland. 



