284 THE AMERICAN MONTHLY [ Oct 
The scheme is excellently carried out by competent 
writers, though the work entailed thereby must have been 
considerable, as the notes run in each case into many 
pages. For instance, a set of twenty slides dealing with 
bacteria is accompanied by a succinct and carefully-writ- 
ten introduction to their study,after which follow detailed 
descriptions of the respective slides, so that the examin- 
ation of each becomes a little lesson in itself, the methods 
of examination and, incertain cases, of preparation not 
being omitted. Another set of twenty slides deals with 
Mollusca, and in the accompanying descriptions we re- 
cognize a well-known writer on marine zoology. The fol- 
lowing extracts will show the nature of these notes. 
Dealing with the palates of Mollusca, the writer says: 
“With but two or three exceptions the mouths of Gastro- 
pods and Cephalopods are furnished with a tooth-bearing, 
ribbon-shaped band, variously known as the radula, odon- 
tophore, lingual ribbon, palate, or tongue; an organ of 
use in scraping, cutting, boring, or masticating, according 
to the habit of the particnlaranimal. It is often of very 
considerable length, and consists of an anterior portion 
working over a cartilaginous swelling, the regular carti- 
lage, upon the floor of the mouth, while the longer hinder 
portion is lodged and formed within a large radular sac, 
which in reality is a deep cylindrical depression of the 
floor of the mouth. When the radular is very long, as in 
the limpet, the radular sac lies free, folded several times 
upon itself, within the body-cavity immediately between 
the viscera and muscular foot disc. Throughout life new 
teeth are continuously added by secreting cells situated 
at the blind end of the radular sac; the singly-refractile 
core of each tooth being secreted by certain celJs upon 
the floor of the sac, while the enamel-like, doubly-refrac- 
tile outer layer is laid on by those of the dorsal wall. | 
As mentioned, that part of the radular which is in use 
plays over a pulley-like cartilaginous cushion, and by the 
