clear and distinct and very large. As we moved the slide along on 

 the stage of the microscope we observed that the shape and size of the 



The 

 to be- 



teeth changed. 



Two rows of teeth on the left side of the radula of Polygyra 

 tridentata, a common land snail. Magnified. (Binney.) 



teeth seemed 

 come broken into 

 several prongs toward 

 the edge of the 

 radula membrane. 

 We called Professor Parker's attention to this characteristic, and he 

 proceeded to explain the radula and how it is studied by scientific in- 

 vestigators. 



" The teeth," he said, " are arranged in five longitudinal rows, each 

 differing from the one next to it. Thus there is a central row, on each 

 side of this, a lateral row; and outside of this, a marginal row. The cen- 

 tral tooth is called the central, or rhachidian ; the lateral teeth are 

 known as admedian ; and the marginal, 

 as uncini. These teeth lie on the 

 radula membrane, and are composed 

 of a base of attachment which is fast- 

 ened to the membrane, and a reflected 

 or turned-over portion which bears the 

 cutting points. This may be better 

 understood by consulting this diagram 

 which I hold before you. In studying 

 the radula through the microscope, care 

 must be taken not to confuse the re- 

 flected portion with the base of attach- 

 ment. This mistake is easy to make, 

 as each has a different focus, the re- 

 flected portion being higher and there- 

 fore nearer the observer. 



" Each tooth in each row is made 

 up of a number of different parts, which, 

 in their diversity, serve to distinguish 

 the different groups of mollusks Thus the central tooth may have a 

 strong, squarish base of attachment to the radula membrane, and the 

 reflected portion may be provided with three cusps or projections a 

 central cusp which reaches to the bottom of the base of attachment, 

 and two side cusps which are shorter. Each cusp may also have a 

 decided cutting point. The lateral and the marginal teeth may also be 

 divided in the same manner. 



41 



Teeth and jaw of land snail. Polygyra 

 pennsylvanica. c, central tooth: 1, lateral 

 tooth: 9, 16, 18, 25, marginal teeth; j, jaw. 

 Greatly magnified. 



