BULLETIN 39, UNITED STATES NATIONAL MUSEUM. [48] 



them. To do this the radula after being careliilly cleaned should be 

 put in a drop of strong solution of chromic acid which stains it yellow 

 brown. It will be seen to change color almost at once, and as soon as 

 dark enough should be taken out and washed clean of the acid. If 

 left in too long the acid may destroy the more delicate parts. It may 

 then be treated as before suggested. 



For a temporary mount glycerine jelly does very well. If Canada 

 balsam is used it rapidly makes the teeth so transparent as to be invisi- 

 ble, and hence mounts in balsam should be stained. The writer has 

 used hydrosilicon, which makes a very good mounting medium at first 

 but in the course of time it becomes obscured by the formation of 

 -crystals. 



The teeth are comprised in three principal longitudinal series, which 

 :are usually easily distinguished, though in some forms one or the other 

 .series may be absent or one lateral series may gradually merge into 

 that next to it. 



ISTormally there is a median longitudinal row of unpaired teeth, one 

 tooth to each transverse row, with the series on either side symmetrical 

 and similar on the two sides with the cusjds inclined toward the center 

 ■of the radula. This central tooth is known as the median or rhachidian 

 tooth. On each side of the median tooth will be found a series of teeth 

 varying in number but separated by their general form, and usually by a 

 toothless space from a set or series of rows nearer the margin of the 

 radula. The teeth of the series near the margin are apt to be small, 

 simple, and similar to one another. In the TrocMdce they are very 

 slender and numerous, almost like fur, and are in each row set upon a 

 continuous solid basis which supports them all. These teeth are called 

 the uncini, though other names have been improperly applied to them* 



The less numerous but usually larger and more diversely formed 

 teeth between the rhachidian tooth and the uncini, are called admedian 

 ■or lateral teeth. In many forms one of these is much larger than the 

 •others and is known as the major lateral, while the others maybe called 

 the minor laterals. The teeth are simple, straight, curved, twisted or 

 compound ; in fact, the diversity is remarkable, but the characteristics 

 for each group, especially genera and families or higher groups, are 

 fally as permanent and important as are the teeth of vertebrates. 



Any series may be wanting. The median tooth is often absent, and 

 in at least one case the two adjacent laterals are ktjown to have become 

 consolidated, making a false or rather a compound median tooth. There 

 are sometimes congenital deformities in the secreting glands. Such 

 individuals may have an asymmetric or deformed radula. The cusps 

 •of the teeth being very brittle are easily broken off by too great pres- 

 sure from the cover glass. The teeth are usually translucent, amber 

 yellow, or dark brown. The cusps, especially of the major lateral, are 

 ^sometimes black on a translucent base, as in Chiton. 



The radula of Litorma and of Patella is very long and narrow. In 



