^SHELLS OF THE LOWER SILURIAN ROCKS. 



635 



of the Old Red Sandstone graduate into the Upper Silurian Rocks, there the contermi- 

 nous deposits contain many similar fossils ; and the same is observed when we descend 

 from the Silurian amid the Cambrian Rocks. We must, however, admit that this lower 

 transition zone is of vastly greater dimensions than in any of the younger formations, 

 and that we are not yet acquainted with its lower limit. 



The examination of all the fossils of the Cambrian System will eventually determine 

 this point, by showing us what new forms appear as we pass through that great slaty 

 series and approach the lowest strata in which the vestiges of life have hitherto been 

 detected. This interesting subject will be cleared up by the labours of Professor 

 Sedgwick. 



FOSSIL SHELLS OF THE CARADOC SANDSTONE AND LLANDEILO FLAGS OR 



LOWER SILURIAN ROCKS. 



PLATES XIX, XX, XXI AND XXII. 



Area Eastnori, PI. 20. f. 1. Transversely ovate, smooth ? 3 very convex; beaks central, short; 

 shell thick ; muscular impressions deep, especially the posterior one ; hinge teeth numerous, 

 slightly diverging. Length 8 lines, width 14 lines ; one imperfect specimen from Golden Grove 

 is nearly If inch wide. 



In the specimens from Eastnor Park, f. 1 a., the casts of the inside show a deep channel 

 bordering the rising corresponding to the posterior muscular impression. In those found at 

 Golden Grove a similar channel, though not so deep, also accompanies the anterior muscular 

 impression f. 1 b. If the latter be not an old shell grown very thick, it may be a different 

 species ; it somewhat resembles the imperfect Area mentioned in the description of the shells 

 from the Old Red Sandstone, 



Loc. In Caradoc Sandstone, near the obelisk of Lord Somers in Eastnor Park ; also near 

 Golden Grove, Llandeilo. 

 Nuculaf Icevis, PI. 22. f. 1. Oval, transversely elongated, ventricose, smooth. Length 3§ lines, 

 width nearly 2 lines. 



Loc. Pensarn, near Caermarthen, in black schist. 

 Avicula orbicularis, PL 20. f. 2 and 3. (Pleurobranchus? G. B. Soiverby in Geol. Trans, p. 268 

 and 340. t. xxxiii. f. 2.) Orbicular, becoming oblong by age, convex, nearly smooth ; beaks 

 prominent; ears small, the anterior one round. Length of the rounder variety 1± inch, width 

 the same. 



Fig. 3. is the form of the old shell or elongated variety from Acton Scott. In some speci- 

 mens the lines of growth form obtuse ridges. Length 2 inches 2 lines, width 1 inch 8 lines. 

 Loc. Horderley ; Cheney Longville ; and Acton Scott, near the Caradoc; also near Ems, 



Nassau ? 



Avicula obliqua, PI. 20. f. 4. Obliquely ovate, elongated, convex, smooth ; hinge line rather 

 short; ears undefined. Length If inch, width nearly as much. 



