240 ON THE PERMIAN CHITONID^E. 



niian strata do not appear to have been widely distributed. Witli 

 the exception of tlie two pLates of a Chiton found at Kirkby 

 Woodhouse, Notts, mantioned by Prof. King-,* and two which I 

 found in the south of Yorkshire, their remains have only occurred 

 in the Permian rocks of Durham. No traces of them have been 

 found either in Russia or Germany. In Durham they are as yet 

 confined to the shell-limestone; and, though the remains of one 

 species have been met with in various localities of this deposit, 

 yet it is only at Tunstall Hill that plates can be obtained in a 

 good state of preservation. Even at Tunstall they are anything 

 but common, and it is only by assiduous search that a series of 

 plates can be obtained. Compared with other localities, however, 

 Tunstall may be said to be rich in their remains, as it certainly 

 is in species, which now occupy an important numerical position 

 in the list of the Permian Gasteropoda. 



It may be mentioned incidentally that the specific distribution 

 of Gasteropoda is greater at Tunstall HiU than at any other Per- 

 mian locality either in England or Germany ; though at the same 

 time it must be understood that even at Tunstall they only form 

 about 32 per cent, of the species of Mollusca found there, while 

 at Ilumbleton Hill, Ryhope Field House, and Dalton-le-Dale, 

 which may be included with Tunstall as principal localities of 

 the shell-limestone, the per-centage is but 16, 15, and 11 resjDCC- 

 tively. And not only is there a paucity as to the species of 

 Gasteropods generally in the fauna of this deposit, but the species 

 which occur are often very rare individually. This is the case 

 at Tunstall ; for there more than one-half the Gasteropods are of 

 rare occurrence, though specimens of Brachiopods and Conchifers 

 are found in great numbers; but, comparatively speaking, it 

 cannot be denied that Tunstall Hill is the head-quarters of the 

 English Permian Gasteropoda. Now this extra-development of 

 Gasteropods in the limestone of this locality seems to imply 

 some difference in the conditions pertaining to the Permian Sea 

 during its deposition, from those which obtained during the ac- 

 cumulation of the limestone at Humbleton, Field House, and 

 Dal ton ; and, as there appears to be no difference in the minera- 



* Monog. rerrn. Foss., p. 203. 



