294 PROCEEDINGS OF UNIlKD STATES NATIONAL MUSEUM. 



Usually, the two inner wings nearly meet one another behind the rhachi- 

 dian tooth, while the two outer ones extend toward each other before the 

 rhachidian tooth. In the unbroken radula, the rhachidian teeth each 

 seem as if enclosed or fenced in by this arrangement of the two adjacent 

 laterals. 



The major laterals show fewer modifications. They are always the 

 largest and most prominent teeth on the radula. They consist of a 

 recumbent shaft, which is partly hollow or excavated behind, crowned 

 by a cusp whose opaque consistency contrasts strongly with the brown 

 translucent shaft and other teeth. This cusp is usually black, or yellow- 

 ish with a black margin. In some species, a x^eculiar areolated spot is 

 visible on the margin, and this may exist in some species in which I have 

 not figured it, as it is diflEicult to observe except with a very strong re- 

 flected light. It does not appear to mark a pore or indentation, but from 

 its constant occurrence in some species must have a certain significance. 



The cusp may be rounded, or ovate, or elongated and simple, or it may 

 be divided into two, three, or four denticles of uniform or varying size* 

 The value of these characters cannot yet be definitely stated ; they can 

 hardly yet be said to present more than specific value, so far as the num- 

 ber of cusps is concerned, yet the general features agree, for the most 

 part, in the same or nearly related groups. 



In LeptocMton, the cusp is greatly elongated, with a small secondary 

 denticle on the inner side, which is abruptly turned up, and, on an ordinary 

 view, resembles a spur or thorn set on the principal cusp. In general, 

 the northern species show a tendency to elongated cusps, simple or 

 divided ; the tropical species, including the typical Chitons, a tendency 

 to a rounded, simple cusp. The majority of all species, however, have 

 a tridentate cusp. 



The shaft and cusp are separated by a distinct line of demarcation 

 where the color changes from black or opaque to translucent. The shaft 

 tapers from the cusp to the lower extremity, which is usually a Uttle 

 expanded. Extending downward from the base of the cusp, the groove 

 or tube in the back part of the shaft is clearly visible. There are thin 

 expansions of the shaft on each side, and sometimes a median keel on 

 the front of the shaft, which in several species is produced into a slender, 

 translucent process, of lanceolate or varied form, extended somewhat 

 inward (toward the rhachidian tooth) and upward toward the cusp of the 

 tooth upon whose shaft it is borne. These processes are most strongly 

 marked in the teeth of the typical Chitons. 



The two inner uncini, between the major lateral and the spatulate or 

 major uncinus, are very irregular in form, even on the same radula. 

 They lie prone on the radula and possess no true cusps, though thickened 

 and elevated into knobs of various form. The major uncinus rises from 

 a very small base, which is twisted and bent under it (as if the tooth 

 was kneeling), and has a twisted, slender shaft, which is expanded at 

 its extremity into a spatulate or feather- formed cusp. The whole tooth 



