THE MOLLUSCAN EADULA. 119 



perhaps with an additional organic substance, form that 

 outer covering- so long known as the enamel layer but 

 hitherto unexplained. I have not been able to confirm 

 Troschel's statement that carbonic acid is present, and though 

 I have made repeated attempts, I have failed to determine 

 whether magnesium is one of the mineral constituents. These 

 points, therefore, must still be left to chemists. The Chiton- 

 idse present us with a deviation from the second type and 

 stand alone among the forms I have examined. In this 

 family ferric oxide is the most important mineral constituent 

 and is the cause of the dark colour of the teeth. 



With the partial exception of Helix aspersa, the ash 

 of the radula preserves the form of the teeth. 



In the first type of radula or that of the Docoglossa the 

 mineral matter may form as much as 27 per cent, of the 

 whole ribbon, this is the case in Patella vulgata; while in 

 the second type it may contribute only 2*4 per cent., as in 

 Helix aspersa, though in this species it sometimes rises to 

 3"3 per cent.; in Dolium galea it amounts, according to 

 Troschel's analysis, to 6 per cent. 



It is interesting to find that the Docoglossa, which are so 

 well-defined a group in other respects, differ so widely, not 

 only from the Pectinibranchiata, but also from the Rhipido- 

 glossa in the composition of the radula. 



We may commence our more detailed account with the 

 Docoglossa, and first with 



Patella. 



If the radular ribbon of Patella is boiled in strong nitric 

 acid, the organic parts are completely dissolved, and there 

 sinks to the bottom of the test-tube a coarse-grained insoluble 

 residue. Microscopic examination shows that this consists 

 of the dark red brown cusps or free biting ends of the lateral 

 teeth (fig. 1), together with some thin, colourless, transparent 

 pieces ; some of these latter are free, some remain attached 

 to the cusps, showing that they are the skeleton of the basal 



