PEOCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 295 



is so twisted and bent that the distal ends of the major uncini, as a rule, 

 are protruded between the cusps of the major hiterals. In a very few 

 species, chiefly of Cryptoidea, the shaft and cusp are abortive, leaving 

 only the small base or knob from which they spring in other species. 

 No very salient characters are afforded by the major uncini. 



The outer uncini have essentially the same characters in nearly all 

 the species. They are flat and scale-like, their edges free and overlap- 

 ping slightly. The outer ones forming the edge of the radula are usually 

 more transverse than the others. In C. articulatus, they are remarkably 

 transverselj" extended. 



It will be seen from this description that, except in their construction 

 and chemical character, the teeth of Chitons are quite dissimilar to those 

 of Limpets, or, indeed, any other described group. 



While not aftbrding grounds for generic distinction by itself alone, 

 the dentition of Chitons, as far as yet investigated, confirms, in many 

 respects, the classification adopted on other grounds by Dr. Carpenter. 

 For instance, his separation of the northern Toniccllw from the tropical 

 Tonicicv of Gray, with which they have usually been united, is fully 

 justified by differences in the dentition. It is i^ossible that when the 

 dentition of the majority of species is determined, some reformation in 

 the limits of subordinate groups may be made i)racticable by its indica- 

 tions, but this is not yet the case. 



The following list of the chief groups recognized by Dr. Carpenter, 

 with the character of the dentition when known, will give a clue to the 

 extent of the work done, and that which is still a desideratum. 



A.— REGULAR CHITONS. 



Leptoidea. 



Leptochiton Gray. (Type L. asellus Lowe.) 



L. cancellatHS Sby. Minor lateral reduced by abortion of the wings nearly to a 

 simple shaft. Cusp of major lateral elongate bidentato; inner denticle much tho 

 smallest, spur-like; shaft simple; other teeth quite simple. Fig. 1, 1 a, showing 

 major lateral from above. X. rugatus agrees. 



Hanleyia Gray. (Type H. dchilis Gray.) 



M. mcndicaria M. & Ad. Minor lateral normal, bi-alate ; major lateral tridon- 

 tate; major uncinus short, other uncini with thickened edges. Fig. 2. 

 DesJiayesiella, Microjplax, and Hemiarthrtim not examined. 



ISCHNOIDEA. 



Traphydermon Cpr. (Tyjie T. cinereus Lowe.) 



T. ruber Lowe. Minor lateral normal, leaf-shaped, with the upper edge of tho 

 outer wing bent over into a sort of cusp ; cusp of major lateral with one large and 

 one small denticle on the inner side, shaft normal; major uncinus short, v.Mth a 

 widely expanded apex with fine radiating grooves on the edge. Fig. 3; 3 a showb 

 the major uncinus from below. 



T. alhiis L. Minor lateral with small wings bent backward ; shaft cusped at tho 

 toj); major lateral bidentate, as in the case of T. ruber, but with the small denticle 

 on the outer side; major nncinus long, spatulate. Fig. 4. 



Trachifradsia, Callochiton, and Stereochiton not examined. 



