48 A MANUAL OF AMERICAN LAND SHELLS. 



stant iu different individuals 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 trans- 

 verse 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, ft, c, f7, is the 

 portion of the tooth which rests ui^on the membrane; I have called it 

 the lase 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 reflection. It is usually tricuspid, 

 the median cnsp h being much longer than the side cusps ff. These 

 last are subobsolete in some species. All the cusps are in most cases 

 ^^'^'•4- surmounted by distinct cutting jyoints ; \ i\s the me- 



cLir""^ ^ ^^ Z diaw cutting point, ^^ the s«de cutting points. These 

 ff—X/j rwv-g cutting points are not always present on the side 



/ wmm~~S\ cusps, and, even when present, are sometimes not 

 ^lllF ** readily detected. Indeed, this is the most difficult 



Central tooth of atrophia •j./'ii j?j.i ii i n^^ 



incana. point of study of the whole membrane. The cusps 



and cutting points vary in development in the various species, and 

 somewhat so in different portions of the same membrane. It must also 

 be borne in mind, while studying my figures of the teeth, that the 

 median cutting jjoint is flat on its lower surface, that is, the surface 

 nearer the base of attachment ; but from thence it first rises and ex- 

 FiG. 5. pands greatly at its sides, and then gradually decreases in 

 size as it still rises and arches over the top. Thus, under 

 the microscope there are two planes prominently seen by 

 changing the focus of the instrument — the plane of the low- 

 est i^ortion of the cutting point and the jilaue of its greatest 

 expansion. In Fig, 5 the former is shown by dotted lines, 

 the latter by the continuous line. In my illustrations the 

 First lateiai of formcr alouc is given. I regret not having shown both, as 

 sus. done by Semper in Phil. Archip., especially as the plane 



*I use the terms xtppcr uiid lower to describe tbe tiynre I give of the base of attach- 

 ment. More properly I sboiihl say anterior aud jiosierior, to describe their positioH ou 

 the membraue, in reference to the head of the laoviug animal. 



tThe cutting points are shaded iu my figures. 



