28 ODONTOSTOMIN^:. 



Finally, Plagiodontes (fig. 2) has the three parietal lamellae united 

 into a single compound tooth, though the'original elements are separate 

 at a certain stage of growth, as shown in pi. 15, fig. 33. In Anos- 

 toma the paiietal lamella is the longest and most deeply entering 

 in the other groups mentioned the infraparietal is longest. 



In other groups of the sub-family, only a single lamella, which 

 seems to be the parietal, stands upon the parietal wall, with the ex- 

 ception of a form of Odontostomus punctatissimus, which retains the 

 minute angular lamella. 



The basal fold is usually situated at or upon the lower end of the 

 columellar margin, rather than in a strictly basal position ; and in 

 many species of Odontostomus the lower palatal fold is basal in 

 situation. In Macrodontes it is rather high. The upper palatal fold 

 is almost always larger than the lower, and is directly opposite the 

 fold of the columella, though frequently above the columellar 

 lamella. In Tomigerus the upper palatal fold is a wide, obliquely- 

 entering plate, and there seems to be no lower palatal. Finally, in 

 most species of Plagiodontes there is a transverse, erect plate within 

 the throat, which may be homologous with the lower end of the en- 

 tering palatal of Tomigerus. 



The following key is based upon obvious structures of the shell, 

 not always the most important generic characters. 



Key to Genera of Odontostomince. 



I. Shell heliciform, biconvex, with the last whorl traversing 

 the base, the aperture turned upward, semicircular and 

 toothed. ANOSTOMA. 



II. Shell turbinate or ovate, wider than high, with a long basal 

 suture ; aperture lateral, the parietal wall bearing angular, 

 parietal and infraparietal lamellse, baso-columellar lip with 

 three folds, outer lip with a single oblique plate within. 



TOMIGERUS. 

 III. Shell ovate or turreted, much longer than wide. 



a. Apex smooth ; aperture narrow, the reflexed outer and 

 inner lips built forward, parallel ; throat without 

 laminae or folds. ANCTUS. 



a 1 . Apex with vertical or grated sculpture ; aperture ovate 

 or oblong, usually contracted by lamellae and folds, the 

 parietal, when present, always compressed and entering. 



ODONTOSTOMUS. 



