358 Dr. H. Hicks — On Bock Classification. 



so far as bodies of the kind can be compared, preserved in sucli 

 different kinds of matrix. 



The points of difference observable are, first, that the valves of 

 " Solenocaris " appear to be longer and narrower than in HeJminthn- 

 chiton, and the lines of growth appear to be hardex', sharper, and 

 squarer, whilst the radiating ribs or strige of the dorsal area are more 

 numerous on the valves of the former than on those of the latter 

 genus. In HelmintJiocMton the lines of growth observable upon the 

 lateral areas of each valve are rather more curved and less quadran- 

 gular than in " Solenocaris." 



Prof. Eupert Jones and Mr. Eobert Etheridge, who have examined 

 and compared the specimens from Thraive, and from Penwhajjple 

 and Balcletchie, are satisfied of their generic, if not of their specific 

 identity. For differences above specified I prefer, on the whole, to 

 treat them as distinct. Under these circumstances, it will be needful 

 to refer them to Helminihochiton and to suggest a fresh specific name 

 for the Chiton from Thraive, and I venture to propose to name it 

 Helminthochiton Grayice, in honour of its discoverer, Mrs. Robert 

 Gray, who has contributed so much to the elucidation of the fossil- 

 remains from the Silurian rocks of Ayrshire, and to whom we are 

 indebted for the opportunity of describing the present interesting . 

 sj)ecimen. 



EXPLANATION OF PLATE IX. Figs. 7-12. 



Fig. 7. Helminthochiton Grayim, H. Woodw. (side-view) natural size ; a. sup- 

 posed anterior or cephalic plate. 



,, 8. ,, ,, ,, (dorsal view) ,, ,, 



,, 9. Two of the valves of same, viewed dorsally, twice nat. size; a, a, posi- 

 tion of apophyses, not clearly to be made out in the fossil. 



,, 10. Side-view of a valve of same, enlarged twice nat. size. 



,, 11. ,, ,, of Helminthochiton {Solenocaris^ solenoides, Young 



and Young (twice nat. size). 



„ 12. Two valves of Chiton squamosus, Lamk. (recent), placed for comparison 

 with the fossil, viewed dorsally. 



Figs. 7-10 from the Upper Bala Beds of Thraive, near Girvan, Ayrshire, in 

 the collection of Mrs. Eobert Gray, of Edinburgh. 



M 



IV. — The Classification of Stratified Rooks. 



By Henry Hicks, M.D., F.E.S., F.G.S. 



R. JUKES-BROWNE'S proposed classification of our rock 

 groups, in the July Number of the Geological Magazine, 

 is a commendable attempt to solve an important question, especially 

 where he proposes to establish divisions on palseontological evidence, 

 in preference to depending upon minor breaks in the succession, 

 which must always be more or less local. 



In some respects, however, his table lacks the definite order 

 and simjjlicity which should be the main point aimed at in any 

 attempt at improving the classification. 



All appear more or less agreed as to the great divisions, and the 

 terms Primary, Secondary, and Tertiary, are on the whole satisfactory. 



