48 NORMAN: ON THE MOLLUSCA OF BERGEN FIORDS. 



mollusca are less and less exposed to be disturbed by tide, 

 storm, or current, and especially where combined with this 

 state of quietude the bottom is a soft, almost impalpable mud, 

 we expect to find and do find that they become more and more 

 slender, thin-shelled and delicate; and this is just what takes 

 place with respect to Dentalium entails. The type form is 

 everywhere in the shallower waters, and in 100 f. and deeper 

 is associated with Z?. abyssorum^ which ranges in 100 — 400 f. 

 in the district examined; but full grown D. agile were only 

 met with at one spot to the south of Bukken in about 200 

 fathoms. Prof G. O. Sars has given a table of the compara- 

 tive length and width of aperture in the three shells. In the 

 following table I reproduce the measurements he gives of 

 the largest shells, and add those of some examined by myself. 



D. entafis. 



Length of Shell. Width at aperture. Anthority. 



I.— 42 Mill 4 Mill. ... G. O. Sars (largest). 



2.-55 >> 5 „ — Bukken, Norway, A.M.N. 



3-— 50 ,, 5 >, ••• Britain (Cumbrae) A.M.N. 



D. striolatum. 



5 >> ••• G. O. Sars (largest), 



4 )> ••■ G. O. Sars (next). 



4 ,, ... Norway, A.M.N. 



4 >> ••• Norway, A.M.N. 



D. agi 



4 „ 



It will be seen from the abo\ 



G. O. Sars (largest). 

 G. O. Sars (next). 

 Sogne Fiord, 670 f.* 

 Bukken, 200 f., A.M.N. 

 Bukken, 200 f., A.M.N, 

 Bukken, 200 f., A.M.N. 



'e that the Bukken specimens 



of 2?. agile ixoxw 200 fathom, which agree with that form in 



* A specimen for which I am indebted to Herr H, Friele. 



J.C, ii., Feb., 1879 



