144 proceedings op the academy of 



The Lingual 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, 1 

 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 oesophagus, 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, 2 but that there 

 are several t3'pes of teeth in the different parts of each transverse 

 row. Three of these t} T pes are found, the central tooth, the teeth 

 on either side of the cental, 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 Zonites inornatus 1 find about 



1 It is very broad in Orthalicus Lignus, some subgenera of Achutinella, 

 some Buli/iiuli, etc. ; in some subgenera of Cylindrella it is very narrow. 



2 Even 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. 



