EXPLANATION OF THE PLATES AND WOOD-CUTS. 



a rounded elevation ; ribs rounded, numerous, about 25 on each side the middle ; 

 beaks remote ; hinge-area broad, curved, its edges nearly parallel. 



This differs from S. hisulcata (M. C. t. 494, f. 1) in its less convexity, more 

 numerous striae, and broader hinge-area ; and from S. bisulcata of Phillips (Geol. 

 of Yorkshire, PI. IX. fig. 14), not of M. C. (which it very nearly resembles), in 

 the greater regularity and number of the ribs *. 



The specimens of this shell are so generally distorted, that its true form is 

 seldom to be clearly made out ; and this, added to the difficulty which already 

 exists of determining between the most perfect specimens of different species, 

 renders it very difficult to ascertain to what species they belong. Perhaps even 

 the S. gigantea from Tintagel (PI. LV. figs. 1 to 4) may be distorted individuals 

 of this species. 

 Loc. Barnstaple and Petherwin. 

 Fig. 14. Orthis interlineata. Suborbicular, compressed, with the front straight, finely 

 radiated, marked with lines of growth interrupting the radii, which increase in num- 

 ber towards the margin ; larger valve not very convex ; its beak but slightly pro- 

 minent ; the smaller valve vith a shallow depression ; muscular impressions nearly 

 parallel in the convex valve, impressions of the hinge-teeth in the other, broad. 



This species, when its internal structure is considered, forms a link serving to 

 connect five species already described in the Silurian System. In the irregularity 

 of its striae, and apparently wide form, it approaches O. lata (Silur. Syst. PI. XXII. 

 fig. 10) ; in the depression and hinge-teeth of the lower or flatter valve it re- 

 sembles the O. orbicularis of the Upper Ludlow (PI. V. fig. 16), and O. canalis of 

 the Lower Silurian (PI. XX. fig. 8) ; and these species are the exact analogues 

 of the O. lunata of the Ludlow (PI. V. fig. 15), and the O. testudinaria of the 

 Caradoc (PI. XX. figs. 9, 10) : all of which, by a careful study of their internal 

 characters, may be at once decided upon when all other characters fail. 



Loc. Petherwin (abundant), Leary, N. Devon, Barnstaple, Morebath, Land- 

 lake, Cornwall. 

 Fig. 15. Atrypa fallax. {Ter. pleurodon. a. Phill. loc. cit. t. xii. figs. 25, 2G.) This 

 species so exactly agrees with specimens of an Atrypa we have from Preston by 

 favour of W. Gilbertson, Esq., that we cannot doubt its identity ; and this appears 

 to be a shell that Professor PhilHps has united with several others, under the 

 name of Ter. pleurodon) it is, however, a genuine Atrypa, though so like the 

 genus Terebratula that we have designated it fallax. 

 Loc. Petherwin, Barnstaple. 

 Fig. 10. Pleurotomaria aspera. Conical, short ; whorls convex with a rounded keel, 

 marked above the band by four principal, and as many intermediate ridges, and 

 beneath it by several smaller ones, all crossed by sharp laminae, which produce a 

 reticulated surface ; band narrow, reticulated. 



* The Terebratulites aperturatus (Schloth. Nachtr. t. XVII. fig, 1) differs from this species in having a 

 strong rib on each side the depression in the lower valve, and in possessing a triangular area. It is abun- 

 dant in the Eifel, and much resembles the shell before us. 



