252 MAETIN F. WOODWARD. 



The Radula as a Whole. — If we recognise the five 

 divisions described above, we may express the arrangement 

 and number of teeth on the radula by the following numerals : 

 ~1, 63, 17, 20, 3; R. 3, 20, 17, 63, 7; there being, as we 

 have seen, a single rhachian, 3 central pairs, 20 lamellate, 17 

 hooked, 63 brush, and 7 flabelliform teeth. 



One of the most noticeable features in this radula, how- 

 ever, is the great difficulty which its teeth offer to our 

 attempts to arrange them in groups, this being due to the 

 presence of intermediate forms between each two adjacent 

 groups of teeth, thus causing them to merge into one ano- 

 ther, and making* it almost impossible to draw any sharp 

 line between them. Nevertheless there are a number of very 

 marked types of teeth in this radula, notably the lamellate, 

 the hooked, the brush, and the flabelliform teeth; of these 

 tiie lamellate and the brush teeth are very striking and 

 peculiar, and not apparently met with in any other mollusc. 



It is somewhat difficult to understand the function of the 

 lamellate and brush teeth, especially the former, and in order 

 to do so we require to know more aboiit the habits and the 

 nature of the food of Pleurotomaria. An examination of 

 the contents of the stomach of two specimens revealed a large 

 quantity of sponge spicules, both megascleres and micro- 

 scleres, belonging to one of the Halichondrina (? a species of 

 Amphilectus). From the fact that many of these spicules 

 appeared to be bound together by tissue, I conclude that 

 Pleurotomaria feeds on the living sponge. For this pur- 

 pose the hooked teeth would be useful in tearing away great 

 pieces of the sponge, and the brush teeth might at the same 

 time rasp away some of the flesh from the spicules; but one 

 is still at a loss to understand the action of the lamellate 

 teeth. 



Another peculiar feature in this radula is the presence of 

 what Bouvier and Fischer term the accessory basal 

 plates. These structures take the form of little chitinous 

 plates, somewhat of the same shape as the basal plates of the 

 teeth themselves, which are attached to the radular mem- 



