Sir R. I. Murcliison on the Silurian Rocks of Cornwall. 339 



of PolpeiTO, Pentuaii, &c., they have been referred to our mutual 

 friend Sir PhiHp Egerton^ who is better versed in the classifica- 

 tiou of Agassiz than anj^ of our countrymen, and he thus writes 

 to me concerning them : — " These remains are 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 Ce- 

 phalaspis or Huloptijchius. The nearest approach is to Bothrio- 

 lepis. The dorsal tin named by Mr. Peach Onchus Murchisoni 

 (Agass.) is not that species, as far as I can determine from the 

 description 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 

 Ctenacanthus. The other Onchus may be O. tenuiserratus, but 

 I have not here the means of comparison. Prom 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 Devonsliire and Cornwall*, and also 

 as being associated A\4th shells, which Mr. J. Sowcrby identifies 

 with the Bellerophon trilobatus (Sil. Syst.) and the Loxomena 

 lincta (Phillips). The first-mentioned of these shells is charac- 

 teristic of the tile-stones hi 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 the Silurian into the 

 Devonian system. Now as Professor Sedgwick and myself had 

 inferred that the limestones of Looe and Fowey belonged to the 

 lower calcareous zone of Devonshire, and as the sections of Su" 

 H. De la Beche show that the Polperro beds dip beneath the 

 Looe and Fowey rocks, the zoological evidences seem to harmo- 

 nize with recorded physical facts, and we thus obtain reasonable 

 grounds for believing, that the lowest Devonian and the upper- 

 most Silurian strata arc exposed in the district which ranges 

 along the shores of that part of Cornwall, by Polperro, Pentuan, 

 &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 disco\ cries of Mr. Peach in the headland of 

 the Dodnian, and in the ])r()longati()n of its strata to Veryan ]3ay, 

 completely demonstrate, that still older and unquestionable Silu- 

 rian rocks are there present. This is the district in which both 



• Professor Pliillips nieiuion.s two very iiiipeifect and tloiiUtf'til scales of 

 fishes, the one in Soutli Devon, liic otlier in North Devon. Palaeozoic Fos- 

 sils, p. 133, figs. 250, 257. 



2 A 2 



