ON THE PERMIAN CH1T0NID.E. 253 



not aware that he adheres to this idea on the authority of new 

 facts, nor that an answer has heen made to the objections of De 

 Ryckholt; so that there still seem good grounds for doubting 

 the relationship of C. (?) cordifer to any group of the Chitonidce. 

 The great paucity of the remains of this subgenus in rocks of 

 all ages, and consequently our want of knowledge of its exist- 

 ence during the past history of life, give considerable interest to 

 the Permian species, which certainly illustrate some of the pecu- 

 liar phases pertaining to its development in palaeozoic times. 



The characters which seem to establish the identity of the fol- 

 lowing Permian species to Chitonellus are the peculiar form of 

 their plates (which indicates their isolated position in the mantle) 

 and the excessive development of the processes of insertion, 

 which were undoubtedly capable of sustaining each plate in a 

 fixed position, though isolated from the others. The form of each 

 plate seems complete in itself, or its shape is such as to be quite 

 incompatible with close connection with others of like form in a 

 united series after the manner of the plates of Chitons. These 

 characters (isolation of plates and large processes of insertion) 

 a,re important peculiarities of the recent Chitonelli, and perhaps 

 form the chief features which distinguish them from the true 

 Chitons ; at least, they are almost the only characters of which 

 a palaeontologist can make use in his determinations of the 

 affinity of the extinct forms. 



The Permian ChitomlU differ from the recent forms in havina- 

 transverse or patelliform plates and peculiar processes of inser- 

 tion, and in other minor particulars. It is not unlikely that all 

 or some of these features were characteristic of the palaeozoic 

 species, which may be imagined to have possessed somewhat 

 different phases from their descendants inhabiting recent seas. 

 But it is premature to offer any speculations on their peculiar 

 characters ; for there are not as yet sufficient materials for 

 generalization, and a comprehensive view must include the 

 Palaeozoic Chitonidce generally. 



VOL. IV., PT. IV. 2 H 



