388 PROCEEDINGS OF THE ACADEMY OF [1893. 



GROWTH CHANGES OF THE RADULA IN LAND-MOLLUSKS. 

 BY DR. V. STERKI. 



The radula, or lingual membrane of the glossophorous mollusca 

 has been most assiduously studied, being a very attractive and 

 interesting object in itself, and a valuable character for systematic 

 distinctions. Its features in the land mollusca of America have been 

 investigated by W. G. Binney ; x but thus far the adult animals only 

 have been considered, and conclusions regarding the morphology of 

 the teeth have been based chiefly upon these. Of the development 

 of the radula and its changes with advancing age, very little is known. 

 The writer published some years ago, a short article on the subject, 2 

 but the considerations then advanced prove to be quite insufficient. 



During the last few months a number of species have been 

 examined with special reference to these changes, and the results 

 prove to be of unexpected value. The subject has been studied from 

 a morphological standpoint only. For histological details the pub- 

 lications of Koelliker, Semper, A. Ruecker and others must be con- 

 sulted. 



The special cases to be described in detail below may be more 

 readily understood by attention to the following general considera- 

 tions. If we carefully examine a well prepared and extended radula 

 of a young specimen, it will be seen that in the anterior, or older 

 part (which was formed, however, by the animal when younger), a 

 transverse row contains fewer teeth than a row in the posterior or 

 more lately formed part. In other words, new teeth, one or more 

 new longitudinal rows, are added. The new teeth are at first mere 

 traces, resembling the outermost marginals in general form, being 

 small, roundish nodules, transverse or longitudinally elongated 

 plates, which, by gradual changes in each succeeding transverse row, 

 develop into true marginal teeth. At the same time it may be 

 found that there are, in a transverse row at the anterior, one or more 

 laterals fewer than in a row at the posterior part ; and a closer 



JTerr. Moll. U. S., Vol. V.; and Jaw and Dentition of Pulmonate Mollusks, 

 Annals N. Y. Acad, of Sci., Vol. III. 



2 Altersverschiedenheiten der Radula bei Hyalinia, Nachrichtsbl. d. Deutschen 

 Malak. Gesellschaft, 1882, p. 172-177. 



