Lept.i:na. 



MOLLUSCA. 



121 



45. OuTHis INTERCOSTATA — Tlio Iiitoi-ribbed Orthis, pi. 

 I.Iir. fig. 43. 



Orthis intercostata. Portlock, Geo. Rep. p. 454, pi. 37, 

 fig. 3. 



Nearly orbicular ; hinge area triangular, its height being 

 equal to about a fourth of the width; surface with fine, thread- 

 like, divergent ribs, about sixteen of which have finer inter- 

 vening stria;, but with those next the umbones equal. 



Silurian Strata, Desertcreat, Tyrone, Ireland. 



46. Orthis lens — The Lens-shaped Orthis, pi. LIII. 

 fig. 44, 45. 



Orthis lens. Phillips, Pal. Fos. p. 65, pi. 26, fig. 110, a, b. 



Suborbicular, much compressed, valves equally convex; lower 

 valve with a mesial sulcus near the umbo, and prolonged in a 

 shallower furrow towards the base; external surface with about 

 twenty undulated striae narrower than the intervening spaces, 

 which are striated transversely. 



Devonian Shales at Hope, near Torquay. 



47. Orthis parallela The Parallel Orthis, pi. LIII. 



fig. 49. 



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

 «, h, c, d. 



Oblong, subovate, much compressed, broadest towards the 

 base; hinge line about half the breadth of the valves; umbones 

 produced ; deeper valve a little tumid on the mesial line, and 

 somewhat hollowed on the other ; subrostral ridges of the 

 deeper valve much lengthened, almost parallel, and including a 

 considerably lengthened, divided oval area ; whole surface 

 covered with numerous, sharp, radiating striae, of unequal length 

 and thickness, with several transverse, well-marked lines of 

 growth. 



Devonian Shales at Pilton and Brushford, North Devon ; and 

 South Petherwin, Cornwall. 



48. Orthis cancellata. — The Cancellated Orthis, pi. 

 LIII. fig. 46. 



Orthis cancellata. Portlock, Geo. Rep. p. 450, pi. 32, fig. 

 19. 



Suborbicular, compressed ; hinge line same width as the 

 valves ; umbones but slightly produced ; whole surface with 

 numerous, fine, somewhat elevated, radiating ribs, about 

 twenty-five in number, with four or five finer intervening, 

 longitudinal stria; ; the whole surface crossed by numerous, 

 fine, concentric striae, producing an elegant cancellated appear- 

 ance. 



Silurian Strata at Desertcreat, County of Tyrone, Ireland. 



49. Orthis interstriai-IS. — The Intcrstriated Orthis, pi. 

 Lin. fig. 48. 



Orthis inlersti-ialis. Phillips, Pal. Fos. p. 61, pi. 25, fig. 

 103. 



Semicircular ; hinge line parallel, and considerably broader 

 than any other part of the valves, forming std)auriform pro- 

 cesses; one valve uniformly convex, the other depressed; 

 whole surface with numerous, sharp, radiating stria:, of unequal 

 length, with very numerous, much finer, intermediate ones ; 

 on the (latter valve the stria; are stronger and more elevated 

 towards the umbo, and assume the character of furrows towards 

 the margin. 



Devonian Shales, Barton, South Devon. 



50. Orthis Calcar The Spur Orthis, pi. LIII. fig. 55. 



Orthis calcar. Phillips, Pal. Fos. p. 138, pi. 58, fig. 112.** 

 Semicircular, a little elongated ; hinge line equal to the 



breadth of the valves; whole surface covered with about twelve 

 granulated, indistinct, divergent ribs, which become strongly 

 marked towards the margins of the valves, and being regularly 

 defined, resembles a fringe all round. 



Devonian Shales at Pilton, North Devon. 



51. Orthis Pecten The Combe Orthis, pi. LIII.* 



%• 1- 



Orthis plicata. Sowerby, Sil. Syst. p. 53, pi. 21, fig. 9. 



Somewhat elongated, compressed ; hinge line flat the whole 

 breadth of the shell; umbones not elevated; sides nearly 

 parallel ; base gently rounded ; whole surface covered with 

 numerous, radiating striae, crossed by many, nearly equidistant, 

 remote lines of growth. 



Lower Silurian Limestone at Coniston. 



Genus IX.—LEPTiENA.— Z)g/;«««. 



Shell equilateral, and inequivalve ; one valve being 

 convex for the most part, and very rarely somewhat 

 depressed; its anterior edge rounded, very thin, deflected 

 or bent downwards, and produced into an irregularly 

 cylindrical form, a little expanded towards its lower 

 edge ; the opposite valve is usually flat, or slightly con- 

 cave on the outside, with its anterior margin reflected, 

 so that its inner edge lies against the inside of the con- 

 cave valve ; the cardinal margin is transverse, parallel 

 and linear, sometimes so much produced on both sides as 

 to give it a winged appearance ; hinge with two some- 

 what tooth-like processes in the upper valve, and the 

 under valve with a crenulated, internal hinge line in 

 most species, which is provided with two elevations, 

 bounding the cicatrices of the muscles. 



1. Lept^na analoga. — The Analogical Lepta;na, pi. 

 LIII.* fig. 10. 



Leptana analoga. Phillips, Geo. York. pi. 7, fig. 10. lb. 

 Pal. Fos. p. 56, pi. 24, fig. 93. Sowerby, Geo. Trans. V. 2nd 

 series, pi. 56, fig. 3. lb. Min. Conch. VIL p. 9, pi. 615, 

 fig. 1. 



Semicircular, compressed ; hinge line generally straight, or 

 sometimes slightly concave, prolonged into pointed auriform 

 processes; lower valve slightly convex near the umbo; flattened 

 on the disk near the base concentrically, and angularly bent 

 towards the upper valve ; surface somewhat corrugated, with 

 flexuous, rounded wrinkles, which are somewhat irregular on 

 the ears ; the whole crossed by pretty equal, rounded, straight, 

 radiating stria;. Length about an inch. 



Found in the Carboniferous and Mountain Limestone at 

 BoUand, Cork, Barnstaple, and Plymouth. 



2. Lept/ENa caperata. — The Wrinkled Lepta;na, pi. 

 LIII.* fig. 7. 



Leptcena caperata. Sowerby, Geo. Trans. V. 2nd series, pi. 

 53, fig. 4. Phillips, Pal. Fos. p. 58, pi. 25, fig. 98. 

 2h 



