1911.] 



NATURAL SCIENCES OF PHILADELPHIA. 



479 



species. Its foundation therefore dates from the memorable Index 

 of Beck, who included some species of other groups. Herrmannsen, 

 in 1847, expressly selected laevigata as the type, probably because 

 this was known to be a Rafinesquian species. Gray, in the same 

 year, names laevigata and olivetorum as examples. Von Martens 

 and American authors have used the name Mesomphix for hetero- 

 geneous assemblages of species, but this later usage cannot be 

 upheld against Herrmannsen' s selection of laevigata as type of the 

 group. 



The dentition in Mesomphix is varied in the several species. 

 M. laevigata is quite peculiar in having no lateral teeth, the inner ones 

 being transitional to the marginal type, and the central tooth is 

 peculiar in the form of the basal plate and the great reduction of its 

 single cusp. The radula in this species is evidently evolving towards 

 the type of Circinaria. 



In the other species the central tooth is well developed and 

 tricuspid, with a somewhat hour-glass-shaped basal plate. This is 

 least marked in M. rugeli, which has teeth much like Omphalina. 

 The number of teeth is also characteristic, as seen in the formulas 

 below, in which the transition teeth are counted as marginals. 



Cen 



M. inornata... 

 M. andrewsce. 

 M. subplana.. 



M. rugeli 



M. laevigata.... 



Mesomphix may be divided into three subgenera, as follows : 

 /. Embryonic whorls of the shell radially striate. 



Subgenus MESOMPHIX. 



No lateral teeth; central tooth with a single small cusp. Flagelli- 

 form gland of the penis very short. Type and only species M. 

 laevigata "Raf. " Beck. 



Subgenus MICROMPHIX (new). 



Several (two to four) lateral teeth; central tooth well developed, 

 tricuspid. Flagelliform gland of the penis long, about half the 

 length of the penis. Type M. subplana Binn. Includes also 

 M . rugeli. 



II. Embryonic whorls smooth or faintly striate spirally. 



Subgenus OMPHIX (new). 



Radula and genitalia as in Micromphix, but embryonic whorls 



