proci:edinos of initkd siaii:s nai kjnal museum '295 



is .so twisteil and bent that tiu' distal ends of ilio major nuc-ini, as a rule, 

 are protruded between the eus});, ot' the majoi' hiterals. lu a very few 

 species, chiefly of Cn/ptoidca, the shaft and cusp are aboilive, leaving 

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

 ^o very salient characters are afforded by the major uncini. 



The outer luicini 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 6'. articiilatioi^ they are remai'kably 

 transversely extended. 



It will be seen from this descriiJtion 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 aflbrding 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 Tonicellw from the tropical 

 Tonickc of Gray, ^ith 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. 

 Leplochiton Gray. (Typo L. asellus Lowe.) 



L. cancellatus Sby. Minor lateral reduced by abortion of tho 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. ilebilis Gray.) 



H. vienclicuria M. & Ad. Minor lateral uonnal, bi-alatc ; major lateral triden- 

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

 Deshayesiella, Microjyiax, and Hcmiarilirum not examined. 



IscnxoiDEA. 



Trachyderinon Cpr. (Type T. cincreiis 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, with a 

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

 the major uncinus from below. 



T. albus L. Minor lateral with small wings bent backwnrd; shaft cusped at the 

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

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



Trachyradsia, ('allochUon, and Stcrcochiton not examined. 



