COOKE : RADULA OF THE VOLUTID^. 



To these the Gwatkin collection adds ten, viz. Cymhium neptuni, 

 Gmel., West Africa, Valuta africana, Reeve, Natal, ferussaci, Don., 

 Patagonia, papillosa, Swains., South. Australia, ponsonhyi, Sm..,y 

 Natal, rutila, Brod., Torres Strait, sophia, Gray, Torres Strait, 

 verconis, Tate, South Australia, Lyria mitrceformis, Lam., Adelaide, 

 quehetti, Sm., ofE Durban. Cymhium diadema, L., ought also to be 

 counted here, as it was figured from a Gwatkin specimen. 



In the Volutidae, the base of the framework of the rhachidian 

 tooth is sometimes almost straight (F. papillosa), sometimes deeply 

 arched (as in Amoria), with every possible gradation of curve between 

 these two extremes. It rarely bulges forward, as in V. musica. It 

 is perhaps desirable to indicate the nature of this curve more precisely 

 than by saying that the base is " slightly " or " deeply " arched. 

 If we imagine the curve, or arc, set upon a chord by drawing a line 

 to join the two ends, and describe angles in the segment thus formed, 

 all these angles are equal. The term " segmental angle " will therefore 

 serve to indicate the nature of the curve in each case, the size of 

 the angle obviously increasing as the curve is less deeply arched, 

 and diminishing as it is more so. In the Gwatkin specimens we have : 



The radula of the Volutidse, as has been pointed out, especially by 

 Dall (5), exhibits, perhaps, more than that of any other family of 

 marine moUusca, a series of progressive modifications from a more 

 elaborate to a simpler type, the series closing with the radula lost 

 altogether. In a few species only {V. concinna, Brod., Valutilithes 

 ahyssicola, Ad. & Roe, Neptuneapsis gilchristi, Sowb.) the radula is 

 normally rhachiglossate, consisting of a median tooth and two 



