262 ON THE PERMIAN CHITONID^. 



Though this species is more nearly related to C. HancocTcianus 

 than to any of its congeners, yet its affinity is scarcely so great 

 as to require particular reference to its distinctive characters. 

 The features distinguishing it from the latter are its smooth sur- 

 face, its median keel, the great size of its process of insertion, and 

 the absence of prong-like apophyses. The plate described last 

 (fig. 30) resembles a plate from the Carboniferous limestone of 

 Belgium, and noticed by M. Koninck under the name of C. con- 

 centricus* 



C. distortus only occurs at Tunstall Hill, in the shell-limestone. 



EXPLANATION OF PLATE XIIL 



Fig. 1. Chitonellus HancocTcianus^ Kirkb}'-, Posterior plate ; a, frontal apophyses 

 of process of insertion ; c, denticulated portion of same process. Mag- 

 nified three times. 



2, 4.———. lutermediate plates of an adult (?) specimen ; a, apophysis 

 of anterior process of insertion ; c, denticulated portion. Magnified 

 nearly four times. 



3. . An intermediate plate of a young individual; showing pro- 

 cess of insertion with apophysis, and denticulated portion. Magnified 

 seven times. 



9, 13. ". Lateral views of an intermediate plate. Magnified. 



10, 12, . Distorted intermediate plates. Magnified nearly four 



times. 



11. . Interior of an intermediate plate, showing the posterior 



process of attachment, h. Magnified. 



5 6, 7, 8. Spines, supposed to have been inserted in the mantle of C. 

 HancocTcianus ; a, processes of insertion. Magnified six times. 



15, 17, 20, 21, 22. Chitonellus antiquus, Howse, sp. Intermediate plates 

 illustrating their variation of outline;/, process of insertion; c, denti- 

 culated portion of the same process. Magnified four and six times. 



18. . Lateral view of an intermediate plate. Magnified four times. 



19. . Posterior plate ; a, frontal apophyses. Magnified four times. 



14. . Lateral view of posterior plate. Magnified four times. 



16. . Interior of posterior plate. Magnified four times. 



23. . Anterior (?) plate. Magnified four times. 



24. Clilton cordatus, Kirkby, Posterior plate ;/, posterior process of inser- 



tion; a a, lateral apophyses. Magnified five times. 



27. . Interior of posterior plate; b, outer groove of anterior 



margin ; cZ, inner groove of the same margin. Magnified five times, 



25, 26. . Intermediate plates. Magnified nearly four times. 



* Descript. cles auim. foss. du terr. carb., p. 322, pi. 22., fig. 4. 



