ON THE PERMIAN CHITONIDJi;. 2GI 



the present it may be described with the above plates as one 

 with an irregulariy rounded outline, a posterior margin elevated 

 centrally and notched where the apex would have been an an- 

 terior margin sinuated in front, with an obtuse angulation on its 

 left side, and a very obtuse one on its right ; a smooth surface 

 marked with strong lines of growth; an anterior process of inser- 

 tion moderately developed, and most produced in front of the 

 lateral angulations; a posterior process, which is thin and in- 

 flexed, bounding the posterior margin. 



Including the latter, I have only taken three plates of tlds 

 species. Neither of the two first-described agree exactly in out- 

 line, though they are alike in general character. In one (fig. 28) 

 the right slope is most developed, in the other the left is the 

 largest; and in the former the posterior margin is somewhat 

 rounded on each side of the apex, and at the apex rather indented ; 

 while in the other the same margin is straight from the apex 

 downwards, and the apex itself prominent. From the very large 

 development of the anterior process of insertion, almost one-half 

 of these plates appear to have been imbedded in the body of the 

 animal. In this respect they rather approach the amiculate type 

 of the CMtomdcG. The strong median keel which divides this 

 process is thickened along its upper edge. I am not aware that 

 this keel or ridge has an analogue in any congeneric species. 



Both the plates of this species and those of C. Hcmcockianus 

 appear to have been subject to distortion; for it may be inferred 

 that figs. 10 and 12 of the latter species, and figs. 28 and 29 of the 

 present, do not represent the normal form of the plates — or at least 

 not in the case of C. Hancockianus, the normal form of whose plates 

 we know, though, in respect to those of C. distortus, it is a question 

 whether the plates found are such as generally characterize it, or 

 are merely accidents, and of comparatively rare occurrence. It 

 is not unlikely, however, that the usual habitat of this species re- 

 sembled that of some of the recent ClutonelU, which are found in 

 tortuous crevices and holes in rocks, twisted out of shape by the 

 confined irregularity of their dwelling-places, from which it is 

 quite impossible to remove them, except by breaking away the 

 rock in which they are imbedded. 



VOL. IV., PT. IV. 2 I 



