46 TERRESTRIAL AIR-BREATHING MOLLUSKS. 



row, or, indeed, of the whole membrane, as all the longitudinal rows, as 

 stated above, have similar teeth. (See Fig. 3.) 



These transverse rows differ in the various genera as to their direc- 

 Fig. 3. tion, either straight, ob- 



lique, or curving, or a 





combination of these di- 

 rections. 



One half of two transverse rows of teeth of Sten. hirsutum. ^ P . , .1 



Of the three types of 



teeth, central, lateral, and marginal, one or more may be wanting. 

 Their number, however, is approximately constant in different individ- 

 uals of the same species, so that, as a specific character, the count of the 

 teeth on one transverse row is usually given ; thus in Zonites inornatiis 

 I find about 23 1 23 teeth, that is, 23 teeth on each side of the 

 central tooth, making 47 teeth in the entire transverse row. 



The characters of the individual teeth vary greatly in the~various 

 genera, especially in some of the genera foreign to our limits. In most 

 cases, however, there are two distinct types of teeth, the quadrate arid 

 aculeate. The former is shown in my figure (Fig. 4). a, b, c, d, is 

 the portion of the tooth which rests upon the membrane ; I have 

 called it the base of attachment. It varies in its proportional length, 

 and in the greater or less expansion of the lower 1 lateral angles. The 

 upper margin of this base of attachment is broadly reflected ; e marks 

 the reflected portion, which I term the reflection. It is usually tri- 

 cuspid, the median cusp k being much longer than the side cusps ff. 

 These last are sub-obsolete in some species. All the cusps are in most 

 Fig. 4. cases surmounted by distinct cutting points;* 



i is the median cutting point, g g the side cut- 

 ting points. These cutting points are not always 

 present on the side cusps, and, even when pres- 

 , . ent, are sometimes not readily detected. In- 

 deed, this is the most difficult point of studv of 



Central tooth of Strophia in- 



the whole membrane. The cusps and ciitting 



points vary in development in the various species, and somewhat so in 

 different portions of the same membrane. It nrnst also be borne in 

 mind, while studying my figures of the teeth, that the median cutting 



1 I use the term upper and lower to describe the figure I give of the base of attachment. 

 More properly I should say anterior and posterior, to describe their position on the mem- 

 brane, in reference to the head of the moving animal. 



a The cutting points are shaded in my figures. 



