" The majority of the teeth of pulmonate mollusks may be divided 

 into two types: first, the quadrate, like a in this diagram of molluscan 

 teeth ; and second, the aculeate, like c in the diagram. The latter 

 has no reflected portion, but a single, thorn-shaped cutting point arises 

 from its sole-shaped base of attachment. 



" In describing the radula, scientific 

 men use a dental formula, similar to that 

 used for designating the teeth of mam- 

 mals. This formula describes the num- 

 ber and position of the teeth and cusps; 

 thus, if a radula has a single central, 

 three lateral, and twelve marginal teeth, 

 and the first series has five cusps, the 

 second series four, and the third series one cusp, the formula will be 



Diagram showing appearance of ra- 

 dula of land snail when seen in section, 

 c, cutting point; ba, base of attach- 

 ment of tooth; r, reflected or turned- 

 over portion of tooth; rni, membrane 

 to which the teeth are attached, called 

 the radula membrane. 



expressed as follows : 



\ 2 



+ T + 



+ 



V 2 



The unit representing the 



tooth is written as a numerator, and the number corresponding to the 

 cusp as a denominator." 



Professor Parker now prepared another radula from a large pond 

 snail, and before mounting it tore it in several pieces with the points 

 of a pair of tweezers and a fine dissecting needle. We asked him 

 what this was for, and he replied that it was necessary to separate 



some of the rows, as well as 

 individual teeth, so that they 

 would be more clearly seen, 

 as they overlaid each other 

 in their natural position, like 

 shingles on a roof. 



We examined the radula, 

 and saw that the teeth differed 

 from those of the land snail 

 in being differently shaped 

 and in having more numerous cusps and cutting points. We also saw 

 that tearing the radula made the teeth stand out more clearly, and 

 they were not so mixed as when the radula was left in its natural 

 condition. 



When examining and making drawings of the teeth of the Mollusca, 

 Professor Parker warned us to be very careful to note every feature 

 accurately. The form of the base of attachment, of the reflection, 

 the cusps, and the cutting points were all of importance. " Don't be 

 satisfied," he said, "with making one examination, make several. 

 After you have made your first drawings and notes, lay them aside 



42 



Diagram of the teeth on the molluscan radula. 

 a, median; b, lateral; c, uncinal or marginal. 1, base 

 of attachment to radula; 2, reflected portion; 3, side 

 cusps; 4, median cusps; 5, cutting points of side cusps; 

 6, cutting point of median cusps. 



