﻿84 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 102 



tooth. On each side of this is a larger lateral tooth. Two more pairs 

 flank the laterals, first the inner marginals, and, lastly, the outer 

 marginals. On the top and leading edge of each tooth are many small, 

 pointed denticles that face anteriorly. These denticles are often used 

 for identification purposes and are generally designated by numbers 

 in malacological works. Hence the formula for the central tooth 



3-1—3 

 (fig. 34, E:CE) is— pr-) indicating that the leading edge bears 1 



large denticle in the center with 3 smaller ones on each side. Below 

 these is a pair of basals, one situated at each bottom and outer corner 

 of the tooth. 



The teeth are transparent and of the consistency of very stiff rub- 

 ber. They are best observed in water between a slide and coverslip. 

 Staining by various conventional means does not appear to bring out 

 any more detail, and, to the contrary, often obscures certain delicate 

 features. Water mounts have the additional advantage of permitting 

 a maximum of manipulation in which the teeth may be further sepa- 

 rated, rolled, or flattened at will by nudging the coverslip back and 

 forth. When dried, the radula may be covered with euparol and the 

 coverslip replace. 



Observations on the radula and the illustrations in figures 34 and 

 35 are from a colony from Naujan River, Mindoro Island, Philippine 

 Islands, P. Bartsch collector (U.S.N.M. No. 258890) . 



The central tooth is oblong and slightly more than twice as wide 

 as high. The usual count on the leading edge is 3-1-3, although 2-1-2 

 is not uncommon. This variation may occur within one ribbon and 

 appears to arise from the changing nature of the cells that form these 

 t«eth. Figure 35, A : E, shows a group of centrals, which anteriorly 

 have a count of 3-1-2, but which progressively reduce the size of the 

 extra denticle on the right side, until 5 teeth posterior to this the count 

 becomes 2-1-2. Examples of additions of denticles were also noted. 

 The maximum number of denticles seen was 4-1-4 but was noted only 

 on one occasion. Not previously noted, and probably overlooked by 

 other workers, are two small basals which are most prominent in 

 centrals of very young individuals (fig. 35, A:H). In older speci- 

 mens these basal denticles are formed nearer the sides and closer to 

 the leading edge and likely have been misinterpreted as being ab- 

 normally placed denticles of the leading edge. 



The lateral tooth is asymmetrical with the denticle-bearing oval 

 section closest to the central. Lateral to this is the armlike extension, 

 which is attached to the hyaline sheath. The denticle count is most 

 frequently 1-1-2, quite often 2-1-2, and rarely 2-1-3. The counts 

 appear to be more stable throughout the ribbon than is the case in 

 the central, but occasionally the left lateral may differ consistently 

 from the right lateral in the same animal. 



