PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 293 



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

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

 from the odontophore, so that only a small portion 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 the form of 

 the individual teeth. They will, like the teeth of Limpets, disintegrate 

 under prolonged .boiling in liquor potassw, so that it is difficult 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 use; those at 

 its posterior termination are of course immature and pulpy ; the scaly 

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

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

 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 oppo- 

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

 tion do not pretend to represent the transverse rows as they appear on 

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

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

 as nearly as practicable, 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 zoological draughtsmen living, for Br. 

 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, considering their small number. 



The rhachidian tooth, as has been stated, always has a simple cusp, 

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

 sinus. The points by which 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 conspicuous manner. 

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



The minor laterals present 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 t hat the sides of a leaf 

 do to its midrib. One or both of the wings may he 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 process 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 partly developed. The outer 

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

 angle with each other, and this an^ie is sometimes less than a right angle. 



