. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology; Zoology. 46 TERRESTRIAL AIR-BREATHING MOLLUSKS. One half of two transverse rows of teeth of Sten. hirsutum. 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. Of the three types of teeth, central, lateral, and marginal, one or more may be wanting. Their number, however, is approximately c
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology; Zoology. 46 TERRESTRIAL AIR-BREATHING MOLLUSKS. One half of two transverse rows of teeth of Sten. hirsutum. 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. 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 inornatus 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 and aculeate. The former is shown in my figure (Fig. 4). a, 6, 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^ lateral angles. The upper margin of this base of attachment is broadly reflected; e marks the reflected portion, which I term the rejiection. It is usually tri- cuspid, the median cusp h being much longer than the side cusps ff. These last are sub-obsolete in some species. All the cusps are in most 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-
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