144 PROCEEDINGS OF THE ACADEMY OF 
THe LINa@uAL MEMBRANE. 
In placing the lingual membrane under the microscope, we at 
once perceive that it is (at least in most of our genera) a long,’ 
narrow, ribbon-like organ, whose whole surface is covered with 
numerous small tooth-like processes, whose reflected apices are © 
pointed, the points directed towards the cesophagus, to which, as 
stated above, they serve to move the food, as well as to perform 
a rasp-like mastication. These teeth are arranged in two series 
of rows, one running longitudinally, the other transversely. 
On careful examination it will be seen that all the teeth of each 
successive longitudinal row are of the same form,’ but that there 
are several types of teeth in the different parts of each transverse 
row. ‘Three of these types are found, the central tooth, the teeth 
on either side of the central, called laterals, and the teeth extend- 
ing from the laterals to the outer margins of the membrane, called 
marginals. 'The change from the single central to the laterals is 
usually abrupt, but from the laterals to the marginals it is usually 
gradual, so that there are several teeth intermediate between the 
two, which may be called transition teeth. The transverse rows 
of teeth are similar on each side of the central tooth, so that it is 
necessary to figure only one-half of one transverse row with its 
central tooth to give an idea of the whole transverse row, or 
indeed, of the whole membrane, as all the longitudinal rows, as 
stated above, have similar teeth. 
These transverse rows differ in the various genera as to their 
direction, either straight, oblique, or curving, or a combination of 
these directions. 
Of the three types of teeth, central, lateral, and marginal, one 
or more may be wanting. Their number, however, is approxi- 
mately constant in different individuals of the same species, so 
that, as a specific character, the count of the teeth on one trans- 
verse row is usually given; thus in Zones inornatus I find about 
1 Tt is very broad in Orthalicus Liguus, some subgenera of Achatinella, 
some Bulimuli, etc.; in some subgenera of Cylindrella it is very narrow. 
2 Hven in case of malformation this holds true. I have often found a 
misshapen, or otherwise abnormal tooth, repeated down the whole length of 
the membrane, or even that a tooth may be entirely wanting in its whole 
length. 
