EXPLANATION OF THE PLATES AND WOOD-CUTS. 



Loc. Petherwin. It appears to be abundant, and very variable in size. 



Fig. 2\. Orthoceras. This specimen, though very imperfect, was figured on account of 

 the very curious longitudinal mark which crosses the edges of the septa, giving it 

 the aspect of the alveolus of a Beleinnite. 



Fig. 22. Orthoceras. A remarkably crushed specimen, so as to resemble a form of 

 Belemnite common in the Lias. Similar occurrences are not unfrequent in the 

 Upper Ludlow Rock of the Silurian series. 



Figs. 23, 24. Fragments of two species of Cahjmene. Fig. 24 much resembles C. tuher- 

 culata (Silur. Syst. PI. XIV. fig. 4.), but differs from that species in the number 

 of lenses to the eye, which are but few in the present specimen. Similar frag- 

 ments are not very uncommon in a soft stone at Petherwin. 



Plate LV. 



Figs. 1 to 4. Spirifera gigantea. Convex, radiated ; sides cuspidate ; radii numerous, 

 proceeding from the hinge-line ; about nine or ten are elevated in the front, which 

 is deeply emarginate ; lines of growth rather conspicuous, crossing the ribs. 



This magnificent species, which occurs nearly nine inches wide in micaceous 

 slate at Tintagel, seems to be intermediate in character between S. disjuncta PI. 

 LIV. f. 12, 13, and S. calcarata, PI. LIII. f. 7. The specimens we have examined 

 are all destitute of shell, and the casts are covered with plates of mica, which lie 

 flat on the surface and give it a glittei'ing aspect ; and they are so distorted in the 

 direction in which the slate has been pressed, that they may easily be taken for 

 many different species. The ribs are about as numerous as in the S. Verneuillti, 

 and are very regular. 



Fig. 1 is a specimen pressed laterally and flattened ; a cast of the interior of the lower 

 valve, natural size. 



Fig. 2 is a young individual, showing the outside of the same valve ; natural size. 



Fig. 3 is a specimen of the same valve, reduced to half its length. 



Fig. 4 shows the upper valve laterally compressed, and folded down upon one of its 

 sides ; natural size. 



We have already given our opinion that this may be the S. Barumensis, but the 

 cuspidate sides may probably entitle it to rank as a distinct species. 

 Loc. Tintagel. 



Figs. 5 to 7. Spirifera costata. {S. speciosa of the Eifel ?) Elongated-fusiform, con- 

 vex, costated ; costae prominent, thick, about five or six on each side the middle, 

 in which two are elevated and approximate ; lower valve with a deep broad depres- 

 sion bounded by two strong ribs ; hinge-area broad, its edges parallel. Length 

 not ith of the width. 



The specimens we have seen of this fine Spirifera are all imperfect, and we 

 cannot speak with certainty as to the surface ; lines of growth appear to cross the 

 ribs, but they may be plaits occasioned by pressure. 



Occurs with groups of corals, fragments, &c., in a soft slaty stone at Fowey, and 

 at Tintagel in a hard blue slate with the former species, and at Looe. 



