KEPOET ON THE POLYPLACOPHOKA. 45 



a monograph on the fossil Chitons, Eochebrune figures x a " valve mediane" of Cryptoplax 

 oculatus, Quoy and Gaim. From its shape it must be a second valve, but it neither 

 agrees with the original description of the valves by Quoy and Gaimard, nor with my 

 specimen, and we are driven to the alternative that it represents the worn valve of another 

 species, possibly of Cryptoplax burrowi, Smith. 



In order to render this account of the Chitonelloidea as complete as my opportunities 

 admit of, I now append extracts from Dall's valuable paper On the Genera of Chitons. 2 



" CHITONELLOIDEA. 



" Tail plate funnel-shaped. Laminae thrown forward." ; Chitonelhcs, Blainv. [Lam.] — 

 Insertion plates very sagittate ; 5 [sic] slits in anterior valve, none in middle and posterior 

 valves ; teeth very short, except at sutures ; eaves distinct ; gills posterior ; sinus very 

 deep and narrow ; girdle, crowded bristles, no tufts ; body very long ; hind valves 

 separate. 4 



The subgenus "Cryptoplax, Gray" [Blainv.], precisely resembles above, except that 

 there are small tufts. 



Choneplax, Carp. — Insertion plates intermediate ; 5 slits in anterior valve, 1 in 

 middle and posterior valves ; teeth moderately long in front ; eaves minute ; gills (?) ; 

 sinus very deep and narrow ; girdle gravelly, with sutural tufts ; valves touching. 4 



On page 288 there is the following definition of this genus: — "Animal repens, satis 

 elongatum : valvae expositae parvae, omnino contiguae ; valva postica iufundibuliformis ; 

 mucro retrojectus, terminalis; laminae ut in Katherina sed obsoletim fissatae ; zona 

 Acanthochitonoidea. Type Cliiton strigatus, Sowerby. West Indies." 



Subgenus Cliitoniscus, Carp. " 49a. Animal et testa Choneplacis similes sed zona 

 baud porifera." 



"Based on Chitonellus striatus and strigatus, Sowerby, Conch. 111., figs. 62, 63, 

 which are represented as without pores. In the former the valves are separated (as in 

 Notoplax) ; in the latter they touch (as in Choneplax)." As Dall truly observes, " The 

 species need examination to confirm the accuracy of the figures." 



The relationship of the Chitonelloidea to the other Chitons has always been a subject 

 of controversy. The tendency of those modern authors who have approached the subject 

 with what may be termed a morphological bias is to regard the former as the most 

 primitive division of the group. 



Professor Hubrecht, for example, says : " This genus [Cryptoplax'] was long looked 

 upon as representing a reduced stage in comparison with Chiton ; different details of its 

 organisation (branchiae, foot, &c.) show the inconsistency of this proposition, and of all 



1 Ann. des Sci. geol. Paris, vol. xiv. pi. iii. fig. 14. 2 Proc. U.S. Nat. Mus., 1881, pp. 279-291. 



3 Op. cit., p. 285. * Op. cit., pp. 289, 291. 



