258 ON THE PERMIAN CHITONID^. 



posteriorly. Intermediate plates in general form not unlike the 

 half of a flat hexagonal pyramid, the frontal margin being three- 

 sided, and the posterior elevated and with a central apex ; from 

 the apex descend four strong ribs, two of which are placed on 

 the posterior margin, the other two are directed towards the 

 frontal margin, and divide the surface of the plate into a central 

 and two lateral cuneiform areas; the central area is steeply in- 

 clined and has a frontal aspect ; the lateral ones are as steep and 

 form obtuse angles with the former; each area is occupied by 

 three or four ribs, which are usually smaller than those bounding 

 the areas ; surface marked by strong lines of growth ; process of 

 insertion of anterior margin produced from within the outer-sur- 

 face margin and rather largely developed; at the frontal angles 

 are two long, prong-like lamellee, or apophyses ; always denticu- 

 lated at one side, generally at the left, but sometimes at the right, 

 never at both sides nor in front ; the posterior process (which is 

 distinct from the former) is formed by the inflexion of the poste- 

 rior margin ; it is triangular in shape, being widest at the apex 

 and wearing away laterally. 



One posterior and five intermediate plates, with a few frag- 

 ments, compose the whole of the discovered remains. Of the 

 latter, two (figs. 10 and 12) appear to have belonged to a dis- 

 torted individual ; one (fig. 3) is very minute, so that most likely 

 it is from a young specimen; the others (figs. 2 and 4) are, per- 

 haps, full-grown plates. The ribs vary in size in different plates, 

 and the apex in elevation. The central apophyses of the ante- 

 rior process of insertion are irregular in length, one being usually 

 longer than the other in the same specimen. From the varia- 

 tion in size and symmetry of these plates, I think it likely that 

 they belonged to three individuals — figs. 1, 2, and 4 to a perfect 

 and probably adult specimen; figs. 10 and 12 to one that was 

 distorted ; and fig. 3 to a young exam]3le. The inconstancy of 

 position in the denticulation of the anterior process of insertion 

 is somewhat peculiar; for it appears rather remarkable that it 

 should vary its position so much as to be found occupying 

 different sides of the plate. The denticulations resemble those 



