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

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

 t'i\y.',\ tlic odontoi)hore, 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 tliat pint of them, especially the two inner uncini, are hidden 

 from view. The radula has to be pulled to jueces, to get at the form of 

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

 under i)ioIonged boiling iji liquor potassce, so that it is difficult to clean 

 the ratlula froiu 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 termiuatiou are of course immature and pulpy; the scaly 

 uucini differ slightly in form with age. The rhachidian tooth is usaally 

 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 api)ear on 

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

 grammatically as they would api)ear if separated from one another, yet, 

 as nearly as practicable, in their relative i^ositions. Only in this way 

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

 drawings, made by one of the best zoological draughtsmen living, for Dr. 

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

 jected, because they lepresented 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 nuuibei'. 



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

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

 sinus. The jjoints by which it is attached to the odontophore are darker 

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

 ance. A side view of the tooth generally i>resents an S shape, and it 

 usually i)r<)jects froih the surface of the radula in a conspicuous manner. 

 The shaft and base have not be(ni observed to present any ornamentation. 



The minor laterals ])resent many modifications of form which may be 

 i-eferred to one type fundamentally. They consist of two parts, a shaft, 

 and wings bearing the same i-elatiou to the shaft that the sides of a leaf 

 do to its midrib. One or both of the wings may be ahnost 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 sometinu's 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 aii^le is sometimes less than a right angle. 



