A MANUAL OF AMERICAN LAND SHELLS. 47 



genera, for instance Bentellaria, the presence or absence of ribs on the 

 jaw is not generic. 



In placing the lingual membrane under the microscope, we at once 

 perceive that it is (at least in most of our genera) a long,* narrow, rib- 

 bon-like organ, whose whole surface is covered with numerous small 

 tooth-like processes, whose reflected apices are pointed, the points di- 

 rected towards the oesophagus, to which, as stated above, they serve 

 to move the food, as well as to perform a rasp-like mastication. These 

 teeth are arranged in two s'eries of rows, one running longitudinally, 

 the other transversely. 



On careful examination it will be seen that all the teeth of each suc- 

 cessive longitudinal row are of the same form,t but that there are sev- 

 eral types of teeth in the different parts of each transverse row. Three 

 of these types are found, the central tooth, the teeth on either side of the 

 central, called laterals, and the fig. 2. 



teeth extending from the laterals 

 to the outer margins of the mem- 



' TfrfTTT'f'^m^^^WfrTTuT^ 





brane, called marginals. The Two tianaYerseiowa of teeth of StrobUalabyrinthica. 



change from the single central to the laterals is usually abrupt, but from 

 the laterals to the marginals it is usually gradual, so that there are sev- 

 eral teeth intermediate between the two, which may be called transition 

 teeth. The transverse rows of teeth are similar on each side of the 

 central tooth, so that it is necessary to figure only one-half of one 

 transverse row, with its central tooth, to give an idea of the whole 

 transverse row, or indeed, of the whole membrane, as all the longitudinal 

 rows, as stated above, have fig. 3. 



similar teeth. (See Fig. 3.) 



These transverse rows differ 

 in the various genera as to their 

 direction — either straight, ob- 

 lique, or curving, or a combi- ,, , ,^ ^ , .. . ., p o, , •; 



' Oneliali of one transverse row 01 tectli 01 /bcrooiia 



nation of these directions. labyrmmca. 



Of the three types of teeth, central, lateral, and marginal, one or 

 more may be wanting. The number, however, is approximately con- 



* It is very broad in Orihalicus, Liguus (see Plate XVI of Terr. Moll., V), some sub- 

 genera of Aohatinella, some BulimuU, &c. ; in some subgenera of Cylindrella it is very 

 narrow. On this same plate I bave given figures of the membranes of the various 

 genera, with a line showing the direction of one transverse line of teeth. 



tEveu in case of malformation this holds true. I have often found a misshapen 

 or otherwise abnormal tooth repeated down the whole length of the membrane, ot 

 even that a tooth may be entirely wanting in its whole length. 



