This genus is readily distinguished from Bulimulus by the sculp- 

 ture of the apex (as in pi. 15, fig. 31). Zaplagius, Oxychona, Otos- 

 tomus, Odontostomus, which have the same apical sculpture, differ in 

 other conchological characters, as well as in dentition. The mar- 

 ginal teeth of Plectostylus resemble those of Drymseus, but in all 

 other Bulimulid genera the form of the teeth is different. It will 

 readily be understood that since the apical sculpture has rarely been 

 mentioned in descriptions, a number of the species not known to me 

 by specimens are placed in this genus provisionally, from their gen- 

 eral resemblance to forms known to belong here. Previous to the 

 discovery that the apical sculpture is correlated with a certain type 

 of jaw and teeth, there was no definite criterion for the classification 

 of the species of Bulimulus and Drymaeus, although von Martens, 

 Crosse and Fischer, W. G. Binney and Pfeffer had recognized that 

 two diverse types of lingual armature were found among the species 

 usually classified as Bulimulus. 



The dentition deserves, and would well repay, an extended study. 

 In some forms, such as D. inter punctus (pi. 60, fig. 16), there are 

 excessively numerous and very minute teeth, in which the basal 

 plate is so thin, and contains so small an amount of conchiolin, that 

 it is scarcely or not visible. The rhachidian tooth has three cusps, 

 the mesocone but slightly larger than the ectocones ; and it is often 

 perceptibly asymmetrical. The rows of teeth run backward each 

 side of the middle, then turn forward (pi. 50, fig. 2). Radulse of this 

 type have been found in D. interpunctus (see pi. 60, fig. 16), D. vir- 

 ginalis (Schako), D. dormani (Binney), palpaloensis and sulphureus 

 (Pfeffer). All of these are thin-shelled species with thin, simple 

 peristome. 



A modification of this type is seen in D. laticinctus, immaculatus, 

 bahamensis, etc., in which the single broad cusp of the rhachidian 

 tooth is notched (see Binney, Ann. N. Y. Acad. Sci. Ill, pi. 12, 

 f. I. H. F.). 



In another series the rhachidian tooth has a single rather narrow 

 cusp (pi. 60, figs. 14, 15, D. vincentinus]. The rows of teeth bend 

 about as in D. interpunctus, but to a less extent, and more gradually. 



In D. arervatus (pi. 58, figs. 75, 77, 79) the basal plates are less 

 indistinct ; the rhachidian tooth is tricuspid, the mesocone much 

 longer than the blunt ectocones. The rows of teeth run slightly 

 backward from the middle towards the margins, and are without the 

 double sinuation described above. 



