PROCEEDINGS 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 laterals. 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 

 transversely 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 affording 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 Tonicellce from the tropical 

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

 justified by differences in the dentition. It is possible that when the 

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

 the limits of subordinate groups may be made practicable 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. cancellatus Sby. Minor lateral reduced by abortion of tlie wings nearly to a 

 simple shaft. Cusp of major lateral elongate bidentate; inner denticle much the 

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

 major lateral from above. L. rugatus agrees. 



Hanleyia Gray. (Type H. debilis Gray.) 



H. mendicaria M. & Ad. Minor lateral normal, bi-alate ; major lateral triden- 

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

 Deshayesiella, Microplax, and Kemiarfhrum not examined. 



ISCHNOIDEA. 



Traphydermon Cpr. (Type T. cinercus 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 largo and 

 one small denticle on the inner side, shaft normal; major uncinus short, with a 

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

 the major uncinus from below. 



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

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

 on the outer side; major uncinus long, spatula to. Fig. 4. 



Trackyradtia, Callochiton, and Slertvchiton not examined. 



