134 IGERNA B. J. SOLLAS. 



and are then described as exhausted. But they now pass on and 

 become the youngest cells of the basal epithelium, shortening 

 till they are of a uniform height with their neighbours. They 

 then travel forward, adhering to the basal membrane and 

 gradually shorten still further. As they continue their course 

 they encounter, as they leave the radular sheath to enter the 

 mouth-cavity, the superior tensor muscle of Amaudrut(2). 

 Now they have to play anew and arduous role: they must 

 adhere to the radular membrane on the one hand and make 

 connection with the tensor muscle on the other, and their 

 tensile strength must be at least as great as that of the pull 

 from the muscle. Strange to say, they nest become liberated 

 from the muscle again and pass forwards, now as a low epi- 

 thelium, until they encounter another muscle, the inferior 

 tensor, when some of them become connected with it. After 

 this they once more move forwards to form part of the walls 

 of a groove (the sublingual groove of Rossler) which is the 

 natural outcome of the mode of growth of the radula and which 

 allows of the free play of the buccal cartilages in eating. 



In conclusion I wish to express ray indebtedness to Professor 

 Sollas for much suggestion and help in carrying out the work 

 contained in this paper, particularly in connection with the 

 observations on the relation between the specific gravity of 

 silica hydrate and its degree of hydration. 



Bibliography. 



1. Adanson. — 'Histoire Naturelle du Senegal,' 1757. 



2. Amaudrut. — 'Ann. Sci. iiat.,' (8), vii, 1898, pp. 1-291. 



3. Akistotle. — ' De Animalibus Historia,' iv, 4, 7, 8, 9. 



4. Beiig]!.— Extract in Troscliel (23) from ' Kouigl. Daiiske Videnskaberncs 



Selkabs Skrifur,' 5t,li Raekke, 3 Bind. 



5. Blocii. — 'Jena Zeitsclir.,' xxx, 1896, p. 350. 



6. Hancock and Embleton.— 'Ann. Nat. Hist.,' 18i5, xv, p. 9. 



7. KoEiiLER.— ' Zeitsclir. Naturw.,' 1856, viii, p. 106. 



8. Lebert. — ' Miiller's Arcliiv,' 1846. 



9. Leuckakt. — 'Arch. Naturg.,' 1851, i, p. 25. 



