116 IGERNA B. J. SOLLAS. 



visible, though sometimes perceptible to the touch. Looked 

 at with the microscope . . . the pointed ends of the teeth 

 are turned towards the stomach like those of the tongue of 

 the lion or cat." Adanson observed the regular arrange- 

 ment of the teeth, and in some cases counted tliem, finding 

 20,000 teeth in 200 longitudinal rows in a bulimoid land- 

 snail which the natives call " Kambeul,'^ and 200 in 10 

 longitudinal rows in Patella. 



Poli (13), in 1791, was the first to give a clear figure of a 

 radula in his magnificent work 'Testacea ufcriusque Sicilife.' 



Troschel (22), in 1836, first established the radula as an 

 organ of great systematic importance. Curiously enough, in 

 the same year van Beneden, in a paper written in 1835, and 

 not quoted in the literature of this subject, points out the 

 possible value of the radula in determining the reality of 

 doubtful species. Troschel's work attracted the interest of 

 zoologists to the radula; after an interval, in which Lebert, 

 AUmau, and particularly Loven, worked along the new lines, 

 Troschel published his ' Gebiss der Schnecken' (1856-1863) 

 — a general and masterly work now well known. His interest 

 was not restricted to the form of the teeth, but extended to 

 their chemical composition. Though Troschel was the first 

 to make the suggestion — thrown out apparently as a shrewd 

 guess — that gi'owth takes place at the posterior end of the 

 radula to make good the waste going on in front, yet he did 

 not follow it up by closer study, nor did he investigate the 

 development of the organ. It has been one of the chief 

 problems of later workers, but they have arrived at some- 

 what conflicting results. By combining a study of the 

 chemical composition with that of the development some of 

 the difficulties which have arisen may be removed. 



Chemical Composition. 



In 1845 Hancock and Embleton (6), in a study of the 

 anatomy of Eolis, state that the radular teeth consist of silica. 

 They base their conclusion on the partly mistaken observation 



