228 PROCEEDINGS OF THE ACADEMY OF [1883. 



liiim of the raclula. These cells do not form the basal membrane 

 directly. It is formed from the many-layered epithelium of the 

 radula. It is not formed, as one would sujipose, by a cuticular 

 secretion of the cells, but at the cost of the cells themselves. The 

 upper layers of the epithelium of the radula coalesce, and thus 

 form the basal membrane. In this manner the epithelium gradu- 

 ally decreases in thickness as it passes forward. Trinchese sa^^s, 

 regarding the formation of the radula, that : "The superior part 

 of the body of each shell is divided into many small rods, which 

 are very small at first and which gradually lengthen as they pro- 

 ceed downwards. These small rods a;-e the denticles. The inferior 

 part of the cell, which takes no part in the formation of the tooth, 

 forms with the similar part of the neighboring cell, the tooth-mass 

 or the true body of the tooth. Final I3' the boundary l)etween the 

 different cells disappears. The nuclei of the tooth-forming cells 

 which remain under the tooth undergo division and give origin to 

 a very compact layer of nuclei, which become more and more 

 pointed as the tooth is shoved forward, are gradually formed 

 in the matrix. When the teeth are so far protruded from the 

 sheath (odontophore), the inferior part of the tooth forms, by 

 means of the la3-er of nuclei, a very resisting cuticle. This cuticle 

 thickens as the tooth advances, while the nuclei or cell-layer 

 gradually diminish in thickness." ^ 



The little rods that he speaks of are not to be found in Helix 

 ajje^ia. As the form of the tongue and the radula is as different 

 in Helix, and further as the tongue-papilla, in the true sense of 



1 "La parte superiore del corpo di ogni cellula, si divide in tauti ])iccoli 

 bastoncelli, i quail, molto costi in principio, si allungano man mano manzan- 

 dosi verso il nucleo 11 quali viene spinto In basso : questi bastoncelli sono 

 i dentinl. La porzlone Inferiore della cellula die non piende parti alia 

 fonnazione dei dentin!, concorre coUa porzlone omologa delie cellule vicine 

 a fonnare il corpo del dente. In fine il limito delie diverse cellule scom- 

 parisce ed il dente e cosi fonnato. I nuclei delie cellule udontogene 

 rimasti sotto 11 dente, si segmentano e danno origine ad uno stratodi nuclei 

 molto spesso, il quale si va assottigliando a secanda die ildente vicne spinlo 

 in avanti dagli altii clii si tbrmans via via uella matrice. Quando i denti 

 sono per uscire dalla guaina, in comincia a formarsi sotte di ossi, per 

 1' attuita dello strato nucleaie, una cuticola molto resistente, la quale li 

 fissasolidameute sul margiue della rotella. Questa cuticola, a seconda che 

 11 dente si spinge in avanti, divene sempre piu spessa, nientie lo strato 

 sottostante si assottiijlia e si esaurice. 



