SHELLS OF THE LOWER SILURIAN ROCKS. 



639 



line equal to the width of the shell; one valve nearly flat. Length 1^ inch, width 1 inch 11 

 lines. 



Having seen specimens of several distinct species sent from Sweden, as O. Pecten, it is with 

 some doubt we refer our shell to that name, 



Loc. Horderley, Caradoc and Meadow Town, Salop ; the Maen, near Meifod. 



Orthis semicircularis, PI. 21. f. 7- Semicircular, slightly produced in front, convex, radiated ; 

 radii 30, sharp, increasing in number towards the margin; beak projecting. Length 3| lines, 

 width | an inch. (It occurs in the Silurian Rocks of North Devon, near Barnstaple.) 

 Loc. South of Bogmine, at Hope, in the Corndon or Shelve Hills. 



Orthis Flabellulum (a), PI. 21. f. 8. Transversely obovate, with about 24 large, rounded, smooth 

 radii ; hinge line less than the width of the shell, slightly arched ; one valve flat, the other 

 convex with a slight channel in the middle. Length 8| lines, width 13 lines. 



In this shell the ribs are remarkably regular, their breadth equal to the spaces between them, 

 and they continue strongly marked to the very beak \ the outline is very regularly rounded and 

 forms an obtuse oval. Can this be O. callactis a. of Dalman ? {L. c. p. 112. t. ii. f. 2.) It 

 appears to have a wide range, descending deep into the Cambrian System. 

 Loc. Corton, Clunbury and other places in true Caradoc Sandstone ; Bala and Snowdon in 

 Cambrian Rocks. 



Orthis Flabellulum (/3), PI. 19. f. 8. This variety is rounder than a. and has a few short radii 

 between the long ones. Length 11 lines, width 14 lines. 

 Loc. East Jlanh of the Caradoc. 

 Orthis virgata, PI. 20. f. 15. The specimens of this species are very imperfect; it resembles the 

 last, but has above 30 in place of 24 radii. Length 8£ lines, width 11 lines. 

 Loc. Acton Scott; Horderley; Llanwyth, Builth. 

 Orthis radians, PL 22. f. 11. Semicylindrical, concave in front, compressed, plaited; beaks ele- 

 vated; plaits about 15, sharp. Length 4^ lines; width £ an inch. 

 Loc. Goleugoed; Llandegley. 

 Orthis? costata, PI. 21. f. 11. Semicircular, with angular sides and about 20 unequal sharp radii. 

 One valve somewhat conical, with a large triangular area beneath the beak. Length 5| lines, 

 width 7i lines. 



Only one imperfect valve has come to our knowledge. 

 Loc. Cefn, near Welch Pool. 

 Orthis Actonice, PI. 20. f. 16. Transversely obovate, with about 14 large radii, trifid or quadrifid 

 at their extremities ; one valve flat, the other very convex. Length 8| lines, width 11 lines. 



Greatly resembling O. Flabellulum, but known at once by the forked radii, a character par- 

 ticularly useful in distinguishing the Species among the slaty rocks. 



Named after Mrs. Stackhouse Acton, in whose property, near Church Stretton, it is abundant. 

 Loc. Acton Scott, Horderley, and the flanks of the Caradoc; also in the Llandeilo Flags, 

 and in the Cambrian Rocks of Bala. 

 Orthis callactis, /3 ? PI. 19. f. 5. {Balm. I. c. p. 113.) Almost circular, but wider than long, 

 nearly flat, ornamented with about 20 rounded radii, which are obscure towards the beaks. 

 Length 7 lines ; width 9 lines. 



The most convex valve of this, is much flatter than the convex valve of O. Flabellulum, 

 - which otherwise it much resembles. 

 Loc. Old Storridge Hill, Worcestershire; Caradoc Hills; Hope Mill, Salop. 



4 M 2 



