ON THE TERMIAN CHITONID.E. 241 



logical character of the Tunstall deposit from those of the locali- 

 ties mentioned to indicate a change of ground, to which we 

 might refer this peculiarity of distribution, and as it can scarcely 

 be ascribed to a mere arbitrary freak of nature in the distribution 

 of species, I am inclined to attribute it to a slight difference in 

 the depth of sea over the Tunstall area. This is not an arbitrary 

 assumption, for there are good reasons for the belief, though it is 

 unnecessary on the present occasion to enter iTirther into this 

 interesting question.* 



In Britain the palaeozoic Chitonidce are as yet confined to the 

 Permian and Silurian systems. No traces of them seem to have 

 occurred in the Carboniferous strata, f nor have any species been 

 described from Devonian rocks. | Prof. King's short notice in 

 1844, of the discovery of a Permian species, was the first account 

 of a palaeozoic Chiton.^ In 1846 Mr. J. W. Salter described a 

 species (Helminthochiton Griffitlii) from the Lower Silurian rocks 

 of Ireland, II and in the Upper Silurian beds of Dudley two species 

 have been found by Messrs Gray and Fletcher, which were de- 

 scribed by Prof. De Koninck in 1857.^ With the exception of 

 the Permian species, these appear to be all that have occurred in 

 the four older systems in Britain. 



On the Continent, several species have been found in strata of 

 the Carboniferous and Devonian eras by Count Munster,** Dr. 



* Thoujrh I still adhere to the tenor of these remarksrit may be added that, since their 

 publication in 1859,1 have found certain univalves more plentiful in a single bed in the 

 quarry at Humbleton. On this horizon Turbo helicinus, Turritella Altenhurgensis, and 

 Straparollus Permiana—tha former somewhat dwarfed— occur in rather large numbers, 

 at least compared with their general scarciiy in other parts of the quarry. (August, 1860.) 



t Since making the above assertion, I have been enabled, through the courtesy of Mr. J. 

 H. Burrow, to examine a small but interesting series of plates of Chiton from the Carboni- 

 ferous limestone of Settle, Yorkshire. The plates appear to belong to four, or perhaps five 

 species, some of which may probably prove identical with species occurring in the Carboni- 

 ferous rocks of Belgium. Others I think are undescribed. A species from the Carboniferous 

 limestone of Ireland has lately been described, under the name of Chiton Thomondiensis, 

 by Mr. W. H. Baily. See Jour. Geol. Soc, Dublin, vol. viii. 



X Prof. De Koninck gives P ymouth a^ the locality of an undescribed Lower Devonian 

 species. See " Bull, de I'Acad. K. de Belg.," 2me se'rie, tome iii., no. 11. 



§ Aim. & ]\rag. Nat. Hist., 1st series, vol. xiv., p. 382. 



II Quart. Jour. Geol. Soc, vol. iii., p. 48. 



^ Bull, de I'Acad. lloyale de Belg., 2me s^rie, tome iii., no. 11. 



*« Beitrage zur Petrefaktenkunde, vol. i., p. 38. 



