126 IGERNA 13. J. SOLLAS. 



Lepeta. 



I take this opportunity of thanking the Rev. Professor 

 Gwatkiu and Dr. 0. Nordgaard for examples of this genus. 



Lepeta coeca. — The siliceous basal pieces of each row 

 are united into a single piece as in C. concentrica^ and as 

 in that species the basal plate is divided by longitudinal lines 

 into six areas, whereas the coloured cusps appear to be 

 formed by the union of four pieces. In this case, however, 

 the double basal piece belongs to the outer cusps. 



Rhipidoglossa. 



Classified according to the chemical composition of the 

 radula, these forms belong to the second type mentioned 

 above (p. 118), in which the proportion of mineral matter in 

 the radula is small and does not include silica. 



In all the forms belonging to the second type which I have 

 investigated the teeth can be isolated from the membrane by 

 cold nitric or hydrochloric acid. Teeth of Trochus zizi- 

 phinus freed in this way and washed were found to have a 

 refractive index lower than 1*557, and higher than r550. 

 The same is true of all the radulas of this type. The 

 refractive index of chitin lies between the same limits. 



Fig. 12 shows the result of staining the radula of Trochus 

 ziziphinus with haematoxylin ; it is necessary to isolate the 

 teeth by teasing, as the membrane stains darkly and the teeth 

 are so close-set that it is impossible to make out details in the 

 radula stained intact. All the teeth take the stain at their 

 roots ; in the series of marginals the length which takes the 

 stain increases as we pass outwards along a transverse row, 

 while, finally, on the outer side of the marginals is a paddle- 

 shaped piece made up of five flat pieces, placed edge to edge 

 (the flabelliform teeth). This piece stains completely, and 

 contrasts with the teeth lying to its inner side. This contrast 

 is most marked in the younger parts of the radula. The 



