THE JAW AND LINGUAL MEMBRANE. 45 



It will be seen below that these peculiarities of the jaw, taken in con- 

 nection with the characters of the lingual membrane, have till now 

 appeared to furnish reliable characters for classification. It must be 

 confessed, however, that exceptions to the usual constancy of characters 

 have been noticed in some genera ; sometimes the difference between 

 strite and ribs is difficult to determine ; sometimes the beak-like promi- 

 nence is greatly modified by a simple median projection. In some 

 genera, for instance Dentellaria, the chai'acter of the jaw is not generic. 



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 types of teeth in the 



J L Fig. 2. 



different parts of each trans- 

 verse row. Three of these 

 types are found, the central 



,, , ,, .., . -, Two transverse rows of teeth of Strobila labyrinthica. 



tooth, the teeth on either side 



of the central, called laterals, and the teeth extending 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 cen- 

 tral tooth, so that it is necessary to figure only one half of one trans- 

 verse row, with its central tooth, to give an idea of the whole transverse 



1 It is very broad in Orthalicus, Liyuus (see PI. XVI.), some subgenera of Acha- 

 tinella, some Bulimuli, etc. ; in some subgenera of Cylindrella it is very narrow. On 

 this same plate I have given figures of the membranes of the various genera, with a line 

 showing the direction of one transverse line of teeth. 



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. 



' ! 



