226 PROCEEDINGS OF THE ACADEMY OP [1883. 



formed b}^ division from cell 5, and dies when the tooth is fully 

 formed, and the remains of this cell are carried forward between 

 the teeth as the radula advances. This can be the only way, for 

 if the cell remained living and continued to secrete conchyolin 

 instead of a series of teeth, we would have simply a solid layer 

 formed on the top of the basal membrane. By a continuous 

 secretion of the cells 1 and 5, the basal membrane moves or is 

 pushed forward, and thus carries the tooth {d) along with it ; after 

 this has proceeded for a short distance (viz., the distance of the 

 space between the teeth), a new cell, which has been formed from 

 cell 5, is ready to commence secreting again, and a new tooth or 

 transverse row of teeth begins to form, and thus the process con- 

 tinues. 



The caps of the teeth are shaded darkly in the figure (PI. X, 

 fig. 5), and are formed after the base of this is completed 

 by cell Jf. The caps are formed by the cells that make up the 

 external layer of the tongue-papilla. If the preparation has been 

 colored with picro- or borax-carmine the basal membrane and 

 bases of the teeth do not color, or only take a slight tinge, while 

 the caps of the teeth are colored darkly. This shows, I should 

 think, that the basal membrane with the bases of the teeth and 

 the caps are of two different formations. 



The covering of the odontophore, which may be called the 

 sheath, consists of two layers. The internal, c' (PI. X, fig. 5), 

 which is made up of a simple laj^er of connective-tissue cells, 

 passes directly into the internal or loose part of the tongue- 

 papilla (e), and it seems that this la3'er is merely a continuation 

 of this part of the papilla. The external layer of the sheath, 

 which covers the whole of the odontophore and is continuous 

 with that which covers the buccal mass, consists of a more com- 

 pact layer of connective-tissue fibres, in which, as in the internal 

 layer, distinct nuclei may be seen. 



In the odontophore the teeth of the radula are directed back- 

 ward. The radula passes from the posterior part of the odonto- 

 phore and extends to the opening in the buccal mass, over the 

 tongue, where it makes a bend and returns on the under surface of 

 the tongue ; the teeth are placed reverse to those on the upper 

 surface, which are directed backwards, while those on the under 

 surface are directed forwards. In fig. 2 (PI. X), I have given a 

 diagramatical longitudinal section of the buccal mass and the 



