118 IGERNA P.. J. SOLLAS. 



Patella, and Dolium were cliosen for study, attention being 

 directed both to the jaw and radula. The radula of these 

 three forms was found to behave in a similar manner and 

 consists of an organic constituent, chitin, together with the 

 inorganic constituents, iron, calcium, carbonic, and phosphoric 

 acid. It was further shown that the radula of Helix 

 nemoralis contains 5 per cent, of ash, that of Dolium 

 galea 6 per cent. 



Koehler's paper, published (7)in the sameyear as Troschel's 

 ' Das Gebiss der Schnecken,' deserves a word of mention, 

 since this observer also affirmed the presence of both an 

 organic and an inorganic constituent, and suspected the 

 occurrence of calcium. 



Later workers continue to make divergent statements in 

 describing the chemical composition of the radula. Sollas (19) 

 in 1885, when studying the nature of the silica in organisms 

 generally, made use of measurements of the refractive index 

 and specific gravity ; he concluded that in the moUuscan 

 radula silica was present and that, as in so many organisms, 

 it was in the form of opal (silica hydrate), but he does not 

 mention the species on which his observations were made. 

 Bloch (5) and others speak of the radula as cliitin, but their 

 views do not appear to be based on original observation. 

 Similarly some modern text-books refer to this organ as com- 

 posed of chitin or conchiolin, others speak of it as siliceous. 

 Huxley and Ray Lankester, with more caution, do not commit 

 themselves on this point. It thus seemed worth while to look 

 once more for definite evidence of the presence or absence of 

 silica and of chitin in the radula. It will conduce to brevity 

 if I state at once the general results I have obtained. I find 

 that in all the odoutophorous Mollusca the radula has an 

 organic basis of chitin ; the Docoglossa are unique among 

 Mollusca in the composition of their teeth, of which the most 

 important constituent is silica hydrate or opal. All the other 

 groups, including the Rhipidoglossa, form a second type in 

 which the radular chitin is hardened superficially by deposits 

 containing calcium, iron, and phosphoric acid, which, together 



