[49] INSTRUCTIONS FOR COLLECTING MOLLUSKS BALL. 



the Scaphopods (Dentalium) it is very short and broad. In most of the 

 land shells the radnla is curved so as to lie flat only under compulsion. 

 In many marine forms the uncini are folded over and lie upon the ad- 

 median teeth, except those which are in active use. 



The study of the radula is of the greatest interest and importance. 

 Auy shell containing the dried animal will probably afford the radula, 

 as the chitine is nearly indestructible. A thorough study of the radula 

 of our common fresh- water shells is still a desideratum and the student 

 remote from the sea will need only to search the nearest ditch to find 

 opportunities for making actual contributions to knowledge. 



The work requires great care, patience, and a certain amount of ex- 

 perience. The beginner will do well to lay aside his first drawings and 

 continue to practice. After a year has iiassed, if he will compare his 

 slides with the drawings first made, he will better appreciate his own 

 progress and the absolute necessity of caution in order to do good work. 



In descriptions of the radula it is customary to use a formula called 

 the dental formula to express the number and situation of the teeth. 

 Each tooth is regarded as a unit and the different series are separated 

 by a colon or period. Thus a radula with one rhachidian, three admed- 

 ian, and twelve uncinal teeth would have as its formula 12 : 3 : 1 : 3 : 12. 

 If it is desired to express the number of denticulations on the cusps of 

 the teeth, the unit representing the tooth is written as a numerator and 

 the number corresponding to the denticles as a denominator, while the 

 fractions representing teeth belonging to the same class but differing 

 in detail are connected by a plus sign ( + ), the series being separated 

 by a colon as before. 



3 1 2 12 13 

 Thus -:-+-:-:--}--:- 

 14 2 3 2 4 1 

 would represent the formula of a radula which had three single cusped 

 uncini, a major lateral with four denticles on its cusp and two bicuspid 

 minor laterals, on each side of a tricuspid rhachidian tooth. Since the 

 admedian and uncinal teeth are bilaterally symmetrical, or similar, on 

 each side of the rhachidian tooth, space is sometimes saved by writing 

 the formula for the lateral or admedian teeth and uncini of one side 

 immediately after that representing the rhachidian tooth. Thus 1 : 

 3 : 3 is the equivalent of 3 : 3 : 1 : 3 : 3. In cases when any of the 

 series are absent their place is represented in the formula by a cipher, 

 so 3 : 3 : : 3 : 3 represents a radula in which the uncini and admed. 

 ian teeth, to the number of three each, are present and the rhachidian 

 tooth is suppressed, 3:0:1:0:3 represents a radula in which there 

 are no admedian teeth and 



1 



: : - : : 



3 



one in which the dental armature is reduced to a tricuspid rhachidian 



series. Further modifications of this principal will suggest themselves 



with experience. 



21387— No. 39 4 



