UNIVALVE UNILOCULAR SHELLS. 241 



at Boulby and Huntcliff. This fossil reaches fifteen inches or more 

 in length, but seldom exceeds four inches in breadth; being an 

 oblong shell, rounded at the base, and greatly resembling a lettuce 

 leaf. As the shell is thin and brittle, perfect specimens can rarely 

 be obtained. Each valve is marked, near the beak, with numerous 

 longitudinal ribs, which grow obsolete before reaching half way to 

 the base, giving place to transverse striae, or lines of growth. The 

 inside is smooth, or slightly waved across, having a pearly aspect ; 

 the beauty of which is increased when the shell is pyritous. The 

 shell is more straight, thick, and rounded, than No. 5 ; but the spe- 

 cimens are usually found much compressed and broken. It is perhaps 

 a variety of the p. rotundata {or gigas ) ; if not, we may term it p. 

 folium. 



A small pinnite, still more rounded, its section being quite cir- 

 cular, occurs in the Staiths beds ; but we have not seen an entire 

 specimen. 



UNIVALVE UNILOCULAR SHELLS. 



Of this division of shells we have no great variety to produce^ 

 for though immense quantities of imivalves occur in the oolite, we 

 do not find so many different genera and species as might be expected. 

 Of the numerous genera, conus, cyprcea, bulla, voluta, strombus, murex, 

 and helix, we have scarcely an instance. A few specimens of cones 

 are said to be found in the oolite. Mr. Parkinson ( III. p. 56 ) men- 

 tions a very fine volute, which he supposed to be from the neigh- 

 bourhood of Whitby ; but we have discovered nothing of the kind. 

 Shells resembling the murex erinaceus, the m. antiqtms, and the m. 

 corneus, occur in the oolite ; but we have not been able to identify 

 them. — A few genera of this class may be noticed as decidedly 

 occurring. 



