218 TERRESTRIAL AIR-BREATHING MOLLUSKS. 



had a single tooth, two had three teeth, and twenty-eight had two teeth, upon 

 the transverse margin, the one nearest the centre being always largest and 

 most prominent ; and all of them had the bilobate, or double curved aperture, 

 and the irregular indentation upon the outer whorl, near the peristome. A sin- 

 gle specimen had three teeth upon the peristome, and three upon the trans- 

 verse margin, making, with two upon the columellar margin, eight in all. The 

 semicircular mouth is abruptly truncated by the last whorl, which forms a dis- 

 tinct and nearly transverse limit. The peristome is thin and a little turned 

 outwards, its edge is often whitish, but within it is brownish, and often thick- 

 ened. The indentation of the last whorl, terminating at the angle of the 

 peristome, is a prominent character. The teeth of the peristome are often 

 curved towards the centre of the aperture. 



The motion of the animal, when in progress, is rapid, but awkward. The 

 proboscis, which is long and projectile, seems to be thrust forward, and attached, 

 and the rest of the foot drawn up to it, reminding one of the motion of a cater- 

 pillar, the shell at the same time rolling from side to side. The adherent forces 

 of the animal evidently lie in the anterior part of the foot. 



This is one of the more aquatic species, and is found under dead leaves and 

 sticks, and on the stems of plants, at the margin of rivulets and ponds. 



The species has been referred to P. antivertigo, but the figure of the dentition 

 of that species given by Lehmann (PL XIV. Fig. 52) does not sustain the 

 theory of identity. 



Vertigo ventricosa, Morse. 



Shell umbilicate, ovate-conic, smooth, polished ; apex obtuse; suture deep; 

 whorls 4, convex ; aperture semicircular, with 5 teeth, one prominent on 



the parietal margin, two smaller on the colu- 

 Flg - * 23 - mellar margin, and two prominent within, con- 



tracting the aperture at the base ; peristome 

 widely reflected, the right margin flexuose, 

 within thickened and colored. Length, .07 inch, 

 breadth, .045 inch. (Morse.) 



Isthmia ventricosa, Moksk, Ann. N. Y. Lye, 



VIII. 1, Figs. 1-3 (Nov. 1865). 



Vertigo ventricosa. Vertigo ventricosa, Mouse, Amer. Nat., I. 669, 



Figs. 61, 62 (186S). — W. G. Binney, L. & 



Fr.-W. Sh., I. 253 (1869). — Tryon, Amer. Journ. Conch., III. 310 (1868). 



— Gould and Binney, Inv., 443, Fig. 705 (18/0). 



Maine, New Hampshire, and New York ; a species of the Northern Region. 



I have not seen this species. Mr. Morse says it has been confounded with 

 V. ovata, but is one fourth smaller, has one whorl less, and a more circular 

 columellar margin to the aperture. 



Jaw wide, narrow, without median projection, but slightly curving at ends; 

 cutting edge regularly waved. 



