Orthis. 



MOLLUSCA. 



Found in the Devonian Shale at Combe Martin, Hagginton, 

 Bradley, and Ogwell. 



Genus VIIL— ORTHIS.— Z)a/»mw. 



Shell inequivalve, with a rectilinear hinge; umbones 

 distant; larger valve with a transverse, smooth area, and 

 a triangular pit. 



Distinguished from Spirifer by the long narrow hinge, 

 and circular flat form of the striated shells. 



1. Orthis Arachnoidea. — The Spider's-web Orthis, pi. 

 LII. fig. 19. 



Orthis arachnoidea. Phillips, Pal. Fos. p. 67, pi. 27, fig. 

 114. Spirifer arachno'idea, lb. Geo. York. 11. p. 220, pi. II, 

 fig. 4. 



Shield-shaped, much compressed ; hinge line nearly parallel, 

 and as wide as the shell; umbones very small, and blunt; upper 

 valve a little convex ; whole surface covered with fine, sharp, 

 divergent striae, which are crossed at intervals by remote lines 

 of growth. 



Mountain Limestone, Haltwhistle, Stradone, Allenheads, and 

 near Hesket, Newmarket, South Devon and Hope, Torquay. 



2. Orthis subarachnoidea The Spider Web-like Or- 

 this, pi. LIII. fig. 27. 



Orthis subarachnoidea. Vernon, Geo. Trans. 2nd series, VI. 

 pi. 36, fig. 3. Orthis arachno'idea, Phillips, Pal. Fos. p. 67, pi. 

 27, fig. 114. 



Elongated; hinge line nearly parallel, and equal to the width 

 of the valves; lesser valve convex; surface with very numerous, 

 continually subdivided and resubdivided striae, arcuated near 

 the hinge, and a few of which are more prominent than the 

 others. 



Devonian Shale, Hope, near Torquay. 



3. Orthis anomala. — The Anomalous Orthis, pi. LIII. 

 fig. 28. 



Orthis anomala. Sowerby, Sil. Syst. p. 638, pi. 21, fig. 10. 

 Anomites anomalus, Schl. Nacht. Pet. p. 65, pi. 14, fig. 2. 



Longitudinally oblong; convex; umbones obtuse; sides nearly 

 straight ; hinge line almost as wide as the valves ; surface with 

 very numerous, longitudinal striae ; base slightly rounded. 

 Length one inch and seven lines ; width one inch and five 

 lines. 



Lower Silurian Rocks, Cardock ; Berwyns and Horderley. 



4. Orthis filiaria The Thready Orthis, pi. LII. fig. 3. 



Spiriferajiliaria. Phillips, II. p. 220, pi. 11, fig. 3. 

 Slightly elongated, nearly pocket-shaped, much compressed, 



narrow above ; hinge line short ; cardinal area very narrow ; 

 umbones greatly blunted; a shallow mesial furrow; surface with 

 rather wide-set, radiating, filamentary processes. 



Mountain Limestone, Holland, Fountains' Fell, Dowall, Der- 

 byshire, and Otterburn. 



5. Orthis connivens The Connivant Orthis, pi. LII. 



fig. 13. 



Spirifera comiivens. Phillips, II. p. 220, pi. II, fig. 2. 

 Subglobose, with an undulated margin; whole surface covered 

 with coarse, waving, divergent, longitudinal striae. 

 Mountain Limestone, Bolland and Pembrokeshire. 



6. 

 2. 



117 



Orthis grandis — The Great Orthis, pi. LII.* fig. 



Orthis grandis. Sowerby, Sil. Syst. p. 638, pi. 20, fig. 

 12, 13. 



Greatly compressed, semioval, short ; with numerous inter- 

 nal bifurcate radiations. Length one inch and a half; breadth 

 two inches. 



Found in the Lower Silurian Limestone at the Flank of 

 Cardoc, Horderley, and Acton Scott. 



7. Orthis elegantula The Elegant Orthis, pi. LII.* 



fig. 3, 4. 



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



Transversely semioval; larger valve very deep, with an acute 

 incurved beak; smaller valve with a concave mesial furrow, and 

 shghtly convex along the sides ; base somewhat pointed, and 

 much compressed ; hinge line a little shorter than the width 

 of the valves ; whole surface covered with fine radiating striae. 

 Length and breadth six lines and a half. 



Found in the Wenlock Shale at Croft; Delves Green; Wool- 

 hope and Falfield, near Tortworth ; and in the Cardoc Sand- 

 stone at Whittingslow, Horderley, &c. 



8. Orthis Virgata The Branched Orthis, pi. LII.* 



fig. 7. 



Orthis virgatus. Sowerby, Sil. Syst. p. 639, pi- 20, fig. 15. 



Transversely obovate, with about thirty rounded, smooth 

 radii ; hinge line shorter than the breadth of the valves, and 

 slightly arcuated. Length eight hnes and a half; width eleven 

 lines. 



Found at Acton Scott; Llanwyth, Builth, and Horderley, in 

 the Lower Silurian Rocks. 



9. Orthis Vespertilio The Bat Orthis, pi. LII.* fig. 



8,9. 



Orthis Vespertilio. Sowerby, Sil. Syst. p. 640, pi. 20, fig. 

 II. 



Semicircular; with numerous, thin, radiating ridges; upper 

 valve a little convex, with a broad, angular, longitudinal, central 

 rib ; lower valve convex, with a deep mesial furrow ; sides an- 

 gular, at the ends of the hinge line. Length three-quarters of 

 an inch ; width one inch and three lines. 



Found in the Lower Silurian Rocks at Cortor, near Clun- 

 burg ; Acton Burnell and Stevens Hill, in the Cambrian Packs 

 at Bala, &c. 



10. Orthis triangularis. — The Triangular Orthis, pi. 

 LH.* fig. 14. 



Orthis triangularis. Sowerby, Sil. Syst. p. 640, pi. 20, fig. 



17. 



Triangular, rounded in front, convex; surface with numerous, 

 fine radiations. Length four lines and a half; width five lines. 



Found in Volcanic Grit, Lower Silurian Rocks at Harring- 

 ton Dingle, near Chirbury. 



11. Orthis Flabellulum. — The Little Fan Orthis, pi. 

 LII.* fig. 39, 40. 



Orthis Flabellulum. Sowerby, Sil. Syst. p. 639, pi- 21, fig. 8. 



Transversely subovate ; hinge line shghtly arcuated, not so 

 wide as the valves; one valve flat, the other convex, with a 

 slight mesial channel ; surface with about twenty-four rounded 

 radii. Length eight lines and a half; width thirteen lines. 



Found in the Cardoc Sandstone at Corton, Clunbury; and at 

 Bala and Snowdon, in the Cambrian Rocks. 

 2g 



