32 THE CHICAGO ACADEMY OF SCIENCES. 
crystal will greatly aid in discovering the radula. This will be 
recognized by its curved, elongate shape and apparently reticu- 
lated surface. If the radula is not found in the first crystal, 
examine the second. When found, in either one of the two 
crystals, transfer to a glass slide and examine under the micro- 
scope with powers ranging from one inch to one-eighth inch. 
It had best be examined by transmitted light. 
After examining and sketching the form of the radula, it 
will be necessary to tear the ribbon up so as to get separate 
rows and individual teeth, since they lie over one another like 
shingles on a roof. The whole of one tooth cannot always be 
seen under one focus, because the recurved cusps of the teeth 
are higher than the base. When the teeth are very transparent 
it will be found necessary to stain the radula. To do this, first 
carefully clean the radula, then put it in a drop of strong solu- 
tion of chromic acid; this stains it yellowish brown. It may 
then be mounted in glycerine jelly. Canada balsam is not a 
desirable medium, since it makes the object tco transparent. 
The teeth are disposed upon the radula in five principal 
longitudinal rows, which are easily distinguished. There is 
always one median longitudinal row of unpaired teeth, with 
several rows on either side. The central tooth is called the 
median or rhachidian. The teeth on either side are called 
laterals or admedian, and when there is an outer row of different 
teeth they are called marginals or wncinmi. In describing the 
teeth notes should be made of the form of the base of attach- 
ment, especially in the central tooth; also other special charac- 
teristics, such as simple, straight, curved or compound. Each 
tooth (as the central) may be made up of anumber of project- 
ing cusps, and great care should be used in describing and 
sketching these. In describing the radula, a dental formula is 
used to express the number and situation 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 would be 
expressed as follows: 
Y+itstit 
The unit representing the tooth is written as a numerator 
and the number corresponding to the cusp asa denominator.* 
*For further information on this subject, the reader is referred to the paper of Dr. Dall 
spoken of above, from which the present information has been drawn. 
