36 Mr R, I. Murchison on the Discovery 



very enigmatical, and I cannot identify a single specimen with any 

 form I know. I do not think any one of the fragments belongs 

 either to Cephalaspis or Holoptychius. The nearest approach is to 

 JBothriolepis. The dorsal fin, named by Mr Peach, Onchus Mur- 

 chisoni (Agass.) is not so, as far as I can determine from the de- 

 scription of Agassiz, unless it be a more perfect specimen than he 

 has seen. The longitudinal ribs, instead of being uniform (as figured 

 by Agassiz), are notched, more after the manner of Ctenacayithus. 

 The other Onchus may be 0. tenuiserratus, but I have not hero the 

 means of comparison. From the general appearance of the collection, 

 I should say they differ from any old Red or Devonian fishes I have 

 ever seen." 



If these ichthyolites do not decisively help us to settle the age of 

 the Polperro zone of rocks, they are still of great interest, as being 

 the only group of fishes worth noticing which has been found in the 

 older rocks of Devonshire and Cornwall,* and, also, as being associ- 

 ated with shells which Mr J. Sowerby indentifies with the Bellerop- 

 hon trilobatus (Sil. Syst.) and the Loxomena lincta (Phillips). The 

 first-mentioned of these shells is characteristic of the tile-stones in 

 Herefordshire and Shropshire, and is also found in strata of the same 

 age in Cumberland (between Kirby Lonsdale and Kendal), which 

 form the uppermost band of the Silurian rocks, or, a transition from 

 Silurian into the Devonian system. Now, as Professor Sedgwick 

 and myself had inferred that the limestones of Looe and Fowey be- 

 longed to the lower calcareous zone of Devonshire, and as the sec- 

 tions of Sir H. De la Beche shew that the Polperro beds dip beneath the 

 Looe and Fowey rocks, the zoological evidences seem to harmonize 

 with recorded physical facts, and we thus obtain reasonable grounds 

 for believing, that the lowest Devonian, if not the uppermost Silu- 

 rian, strata, are exposed in the district which ranges along the shores 

 of that part of Cornwall, by Polperro, Pentaun, &c. 



But, if doubts should exist as to whether the Polperro slates ought 

 to be referred to the bottom of the Devonian or top of the Silurian 

 system, the discoveries of Mr Peach in the headland of the Dodman, 

 and in the prolongation of its strata to Veryan Bay, completely de- 

 monstrate, that still older and unquestionable Silurian rocks are there 

 present. This is the district in which both Professor Sedgwick and 

 Sir H. De la Beche had noted the existence of a line of elevation, f 

 running from NE. to SW., which, bringing up certain quartzose or 

 argillaceous slates, had thrown the beds oiF, both to the NE. and 

 SW., the published section of the latter having clearly indicated 

 these relations. 



* Professor Phillips mentions two very imperfect and doubtful scales of 

 fishes, the one in South Devon, the other in North Devon. Palae. Fossils, p. 133, 

 figs. 25G, 257. 



t See Trans. Geol. Soc, N.S., vol. v., p. 663; and Report on Cornwall and 

 Devon, p. 84. 



