ON THE PERMIAN CHITONID^. 239 



could not but conclude that the latter belonged to the same 

 subgeneric group. This discovery originated a suspicion of 

 the supposed generic affinities of my specimens of Calyptrcea 

 antiqua. The next day I examined the latter again, and found 

 that there could be little doubt that they belonged to Cliiton- 

 ellus likewise, as well as Mr. Howse's type- specimen, the 

 identity of which with my specimens was indisputable. These 

 interesting discoveries caused me to search the limestone of 

 this locality for further specimens ; and, though they are very 

 scarce, I succeeded in finding a few additional plates, which 

 have greatly increased our knowledge of the details of the species. 

 On dividing the plates according to species, I found it necessary, 

 after making due allowance for individual variation of form, to 

 refer them to four specific groups. One of these, of course, re- 

 tains the specific name given to it by Mr. Howse, its original 

 discoverer. The others seem to be unknown, and are described 

 as new. 



The distribution of the Chitonelli is extremely local at Tunstall 

 Hill. All the specimens found occurred in one of those curious 

 cavities of calcareous dust which I have lately noticed in de- 

 scribing some Permian Entomostraca.^ It is owing to their hav- 

 ing occurred in this matrix, and their consequently easy extraction, 

 that I have been enabled to work out the details of the species 

 more perfectly than could otherwise have been the case. Along 

 with them were found great numbers of single and united valves 

 of Spiriferina multiplicata, and fragmentary remains of Cidaris 

 Keyserlingi and Cyatliocrinus ramosiis. Three of the plates of 

 Chiton cordatus, one of the new species, occurred in the usual 

 semi-crystalline limestone of the locality. Nearly all the specimens 

 of the older species were found in this harder matrix, and for this 

 reason they are difficult to extract without injury. It is ex- 

 tremely rare to find specimens of these species that illustrate the 

 internal surface of the plates ; and it is principally from casts that 

 our knowledge of the interior has been obtained. 



As a group, the representatives of the Chitonidce in the Per- 



* Ann. & Mag. Nat. Hist., 3rd series, vol. ii., p. 318 ; and Trans. Tynes. Nat. Field Club, 

 vol. iv., p. 122-171. 



