244 ON THE PERMIAN CHITONIDiE. 



arcuate longitudinally, with an apex posteriorly situate and more 

 or less produced in different plates ; posterior margin rather con- 

 cave; extremities rounded; anterior margin somewhat convex; 

 lateral areas prominent, curved, narrow; dorsal area ornamented 

 with an elevated, flatly rounded, longitudinal ridge, which increases 

 in width anteriorly, and is bounded on each side by a slightly 

 divaricating sulcation ; the same area is crossed by two oblique 

 angulations proceeding from the apex to the anterior margin; 

 surface covered with strong lines of growth, most conspicuous 

 on the lateral areas. Posterior plate capuliform, rounded medianly ; 

 apex bluntly pointed, very posteriorly situate, reflexed, and 

 rather depressed, projecting beyond the extreme margin; pos- 

 terior margin semicircular; anterior margin deeply sinuate dor- 

 sally, convex laterally; surface marked with strong lines of 

 groAvth, and diagonally angulate from the apex to the point 

 of juncture of the margins. The jirocesses of insertion of the 

 intermediate and posterior plates are strong and of a nearly 

 semicircular form; they originate on the inner surface of the 

 anterior margin, near the lateral extremities, and reach to within 

 a third of the median line ; a thin inflexed process of attachment 

 (rather than of insertion) bounds the under surface of the posterior 

 margin. 



It may seem somewhat presum.ptuous in me to offer a fresh 

 description of a species which has already been described ; and 

 it must be understood that I do so with due deference to the 

 labours of those who have preceded me in the investigation of 

 the characters of this Chiton. I might have adopted the diag- 

 nosis of Prof. King,* or the later description of Mr. Howse;f 

 but, as the former is subject to modification, owing to the dis- 

 covery of additional details, and as I cannot subscribe to all the 

 opinions enunciated in the latter, I have thought it best to 

 describe it afresh. 



Plates belonging to young individuals have the median line 

 sharper than in full-grown specimens ; the same feature is also 

 less arched longitudinally in such plates, and the lines of growth 



* Monog. Perm. Foss., p. 202. 



t Ann. & Mag. Nat. Hist., 2 ser., vol. xix., p. 463. 



