PEOCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 293 



The teeth of the Chitons are excessively difficult objects to make ont, 

 though some of the species are quite large. The teeth project strongly 

 from the odontophore, so that only a small ijortion of any one tooth can 

 be had in focus at one time. Moreover, they overlie one another to such 

 an extent that part of them, especially the two inner uncini, are hidden 

 from view. The radula has to be pulled to pieces, to get at tlie form of 

 the individual teeth. They will, like the teeth of Limi^ets, disintegrate 

 under i)rolonged boiling in liquor potassce^ so that it is difldcult to clean 

 the radula from adherent mucus or remains of food. The teeth on the 

 anterior edge of the radula are always worn or broken by usej those at 

 its x^osterior termination are of course immature and pulpy ; the scaly 

 uncini ditfer slightly in form with age. The rhachidian tooth is usually 

 more or less embraced by the wings of the minor laterals, so that it aiJ- 

 pears as if set on a plate or in an open box, and must be disentangled 

 before its form can be made out. 



In these descriptions, the front of a tooth is taken to be the side oi)po- 

 site to that by which it is attached to the radula. The figures of denti- 

 tion do not x)retend to represent the transverse rows as they appear on 

 the unbroken radula. On the contrary, the teeth are represented dia- 

 graumiatically as they would appear if separated from one another, yet, 

 as nearly as i)racticable, in their relative positions. Only in this way 

 could any idea be given of their forms and number. A series of exquisite 

 drawings, made by one of the best zookigical draughtsmen living, for JDr. 

 Carpenter, nearly led me into serious error, and have been totally re- 

 jected, because they represented only what could be seen without dis- 

 membering the radula. The diagrams given, if somewhat rude, are, it 

 is believed, tolerably reliable, and the result of a surprising amount of 

 work, considermg then- small number. 



The rhachidian tooth, as has been stated, always has a simi)le cusp, 

 which may possess a somewhat sinuous edge or a tendency to a median 

 sinus. The i^oiuts by w^hich it is attached to the odontophore are darker 

 than the rest, and, seen through the translucent shaft, modify its appear- 

 ance. A. side view of the tooth generally presents an S shape, and it 

 usually projects from the surface of the radula in a consiucuous manner. 

 Tlie shaft and base have not been observed to present any ornamentation. 



The minor laterals ju'esent many modifications of form which may be 

 referred to one type fundamentally. They consist of two parts, a shaft, 

 and wings bearing the same relation to the shaft that the sides of a leaf 

 do to its midrib. One or both of the wings may be almost abortive, 

 leaving only the shaft twisted into a cusp at its apex, or the edges of 

 the wings may be bent over into a cusp at the top of the tooth, and a 

 small i^rocess like a bud or button is thus sometimes formed on the outer 

 upper angle of the tooth. The most common form is that where the teeth 

 are somewhat leaf-shaped, with both wings i^artly developed. The outer 

 wing aborts before the inner one. These wings meet the midrib at an 

 angle with each other, and this au^i^le is sometimes less than a right angle. 



