480 Miss I. B. J. Sollas. On the Identification of [May 27, 



from the basal membrane, shows that this cannot be so. The same radulse 

 of which I had measured the specific gravity were treated for a moment 

 with strong hydrochloric acid, with the result that the basal membrane was 

 instantly destroyed and the teeth freed; after washing and reboiling in 

 alcohol they were found to have specific gravities of even wider range than 

 that of the intact radula, the heaviest of them being 1*430. By soaking the 

 radula of Buccinum for several weeks in very strong potash (above 

 40 per cent.) the teeth can be isolated, the basal membrane being destroyed. 

 This fact, discovered by chance, aroused some doubt as to whether the basal 

 membrane were really chitin, and a repetition of the experiment was made 

 in which the skeletons of two segments of Astacus were subjected to the 

 action of the same strength of potash. The two segments became isolated 

 from one another, the abdominal feet (pleopoda) were also isolated from the 

 segments, the soft connecting membranes being destroyed. And yet these mem- 

 branes have the same specific gravity and the same refractive index as chitin, 

 and no one has ever suggested that they are anything else. These membranes 

 differ in staining reactions from the chitinous pieces which they connect in just 

 the same way as the basal membrane of Buccinum differs from the teeth. 



A third method of isolating these teeth is to soak the radula in hydro- 

 fluoric acid for 24 hours previous to the usual cleansing process. The teeth 

 will then be liberated when the radula is boiled in 5-per-cent. potash 

 solution. Teeth thus prepared gave the least range that I have found in this 

 material ; they formed in all cases a dense zone at the level 1*393 ; in one 

 instance they were confined to this zone, but in two other columns a certain 

 number of teeth were scattered down to 1*411, and a very few, perhaps five 

 or six, hung at various depths below this. Astacus integuments subjected 

 to precisely the same treatment had a specific gravity of 1*406 ; and, as 

 happened when strong potash was used, the connecting membranes were 

 destroyed. Some of the heavier Buccinum teeth were removed with a 

 pipette from the column ; in one or two of them a single large granule, 

 apparently a foreign body, was visible : it appeared to have undergone partial 

 solution. Since, in the case of Murex, I have seen obvious sand grains 

 firmly embedded in the teeth of the radula, it would seem probable that the 

 exceptionally heavy teeth are to be accounted for by accidents of this kind. 

 The minor variations in specific gravity may perhaps be due to some second 

 organic substance accumulated in the thick body of the tooth remote from the 

 margin, and thus not affecting the measurement of the refractive index, which 

 lies constantly between 1*550 and 1*554. The fact that the heavier teeth hang 

 in the column with their fangs upwards is in accordance with this view. 



The refractive indices of the organic portion of the Molluscan radula, 



