NATURAL HISTORY SOCIETY OF GLASGOW. 85 



Chitonidae in the Carboniferous rocks of Scotland. Of Chiton 

 proper, two species have been met with in the Upper limestone 

 group, and one in the Lower group. Of Chitonellus, two, both in 

 the Lower limestone group. In the present communication 

 additional species of Chiton, and four of Chitonellus are described, 

 all from the Lower limestone group. 



The Carboniferous rocks of Belgium have hitherto yielded the 

 largest number of forms, if we accept all the species described by 

 Baron de Eyckholt. Now, however, both the Permian system of 

 England, and the united Carboniferous series of Great Britain will 

 compare favourably with the above. 



2. DESCRIPTION OF THE SPECIES. 



Genus Chiton, Linnaeus. 



Obs. — Too little is at present known of the entire structure and 

 appearance of the Palaeozoic Chitons to warrant any attempt at 

 generic sub-division, such as is adopted amongst the recent species. 



In the recent forms the soft parts have played an important part 

 in the classification of the family, although, to some extent, the 

 exoskeleton has been taken into consideration. To the Palaeontolo- 

 gist the latter is naturally the only part of the Chiton-economy 

 which needs consideration, but, as in the recent forms, this affords 

 anything but a satisfactory basis. The following are the only 

 palaeontologies! sub-divisions with which I am acquainted. 



1. Helminthoehiton, Salter, 1847. (Quart Jour. Geol. Soe., 

 iii., p. 51.) Proposed for a Silurian type, in which the plates are 

 " deeply emarginate behind, and, as it were, bent backwards," a 

 character very rare in living species. The apex is on the posterior 

 edge, and without any expansion behind it. The full characters 

 are as follows : — 



" Elongate, plates as long as wide ; subquadrate, thin ; apex of 

 the anal plate remote from its front edge ; sustentacula widely 

 separated ; shell but very little covered by the mantle/' &c. 



The genus was divided by Salter into three groups, the genus 

 proper, typified by the species II. Griffithi, Salter, and two other 

 unnamed groups, containing mostly the Carboniferous species. In 

 all probability Helminthoehiton will be found to be a satisfactory 

 receptacle for such forms as the type species, but the sections will, 

 1 think, scarcely stand the test of investigation. 



