96 PROCEEDINGS OF THE 



Mr. Kirkby has called attention to the peculiar form of the plates 

 of the Permian Chitonelli,* and the great development of the 

 processes of attachment inserted in the mantle. We now know 

 similar patelliform plates of Carboniferous age, although there does 

 not appear to be so large an amount of inserted surface in the latter, 

 as in the former. In Chitonellus the relatively larger proportion of 

 free mantle surface to that occupied by the plates is something 

 remarkable. 



The plates are placed along the median line, usually separate 

 from, and each independent of, the other. In a few forms, however, 

 only the posterior plates are free and separate, whilst the anterior 

 are close to one another, and there is a partial overlapping, indicating 

 a return to the type of an ordinary Chiton. A noticeable feature 

 appears to be the difference in size of the exposed portions of the 

 plates, and that inserted in the substance of the mantle, the latter 

 being, as a rule, much larger than the former, which is usually highly 

 ornamented. 



Chitonellus subquadratus, Kirkby and Young. 

 (PI. II., figs. 4 and 5). 



C. subquadratus, K. & Y., Geol. Mag., 1867, iv., p. 342, t. 16, f. 5. 



Sp. Char. — Intermediate plate subquadrate in outline, much 

 arched, and acutely carinate along the middle line ; anterior and 

 posterior margins excavate, the former much the most so, the 

 posterior excavation being comparatively shallow. The inserted 

 portion of the plate is greatly developed as compared with that 

 exposed without the mantle. The latter resembles a more or less 

 heart-shaped excrescence, with the point of the heart directed for- 

 wards ; at the posterior end is a raised and slightly prominent mucro ; 

 the lateral margins are well defined, and within their border occur 

 scattered granules gradually becoming smaller inwards, but not 

 extending over the mucro. The general surface of the inserted 

 portion of the plate around the margins is marked by a series of 

 diverging somewhat granular ridges. 



Obs. — I am happy in possessing the support of Mr. Kirkby's 

 valued opinion in referring this plate to Chitonellus subquadratus, 

 K. and Y. It appears to differ only in the absence of the two an- 

 gular diverging ridges at the posterior end, and in the possession of 

 a more highly developed mucro on the exposed portion of the plate. 

 * Quart. Jour. Geol. Soc, xv., p. 618, 



