202 A MANUAL OF AMERICAN LAND SHELLS. 



nuscnlus. Some species have vertical stria3, especially on the middle of 

 the jaw (Fig. 211). Some have strong transverse lines of reinforcement 

 (Fig, 212). In several species, such as Z. viridulus and Z. Binneyanuft^ 

 Morse has detected projecting points on the cutting edge of the side 

 of the median beak, but I did not find them in a specimen of the last 

 species examined by me. The jaw of this last species is very high. 

 Tliat of Z. cxigiius \s very low. The median vertical grooves in some 

 species have already been mentioned under Z. fcrreus and milium. 



In the genus Glandina we found only the aculeate form of teeth, or 

 pure marginals; in MacrocycUs we found, in addition to these mar- 

 ginals, a few teeth showing a modification of this type, being the tran- 

 sition teeth from marginals into laterals; in the present genus, Zonites^ 

 we find for the first time the lateral teeth in their full developement. 

 Thus we have usually the three forms of teeth — centrals, laterals, and 



Flc. 214. 



I 



.1 

 General view of dentition of Zonites arboreus. 



marginals — all present, and apparently a generic characteristic. It will 

 be noticed however, that in Iwvigatus * (Terr. Moll. Y, Plate II, Fig. 

 F) there is no perfect lateral, the first tooth showing a decided modifica- 

 tion or transition into the marginals. Thus we cannot say that in all 

 species of Zonites there are pure lateral teeth. It will be seen below 

 that in some species the number of laterals is reduced to two. 



I give in Fig. 214 a general view of the arrangement of the teeth in 

 Zonites.] The centrals have a base of attachment longer than wide, sub- 

 quadrate, with lateral expansions at the corners of the lower margin. 

 The reflected portion varies in size in the various species, from highly 

 developed in viridulus and others to slightly developed in lasmodon and 

 others, in the latter case resembling the short reflection of Vitrina. 

 The reflection always bears a more or less developed central cusp, gen- 



* See also Z. cellarius. 



tThe characters of the separate teeth of this species are better shown in Plate III, 

 Fig. F, of Torr. Moll., V. 



