496 LiMN^iDJi:. 



Planorhis exacutus, Sat, Journ. Acad. Nat. Sc. ii. 1G5 (1821); Bixnt.y's cJ. C4. — Hal- 

 DEMAN, Mon. 21, pi. 4, figs. 1-3 (1844). — Gould, Inv. of Mass. 208, fig. 137 

 (1841). — Adams, Shells of Vt. 155 (1842). —Ue Kat, N. Y. Moll. 03, pi. 55, fig. 

 62a, 6 (1843). — Anox. Can. Nat. ii. 207, fig. (1857). — AV. G. Bixxey, Smith. 

 Inst. L. and Fr. W. Shells, ii. 126, figs. 210-213 (1865). 



Planorbis lens. Lea, Tr. Am. Phil. Soc. vi. 68, pi. 23, fig. 83 ; Ohs. ii. 68 (1839). 



Planorbis Brogniartiana, Lea, Tr. Am. Phil. Soc. ix. 24; Oljs. iv. 24 (1844) ; Vr. ii. 242 

 (1842). 



Planorbis lenticularis, Lea, Tr. Am. Phil. Soc. ix. 6 ; Obs. iv. 6 (1844). 



Planorbis Burhanensls, Lea, Tr. Am. Phil. Soc. ix. 6 (1844) ; Pr. ii. 32 (1841) ; Obs. iv. 6. 



Paludina hyalina, Lea, Tr. Am. Phil. Soc vi. 17, pi. 23, fig. 81 ; Obs. ii. 17 (1839). 



Planorbis exacuous, Say, Journ. Acad. Nat. Sc. ii. 166. 



Shell lenticular, light transparent horn color ; whorls four, flat- 

 tened so that the width of each is at least twice its depth, the upper 

 and lower surfaces convex, and brought to a sharp exterior 

 ^^' edge; the last half of the outer whorl deflected, so that 

 p. exacu- the termination of the sharp edoe is on a level with the 



tus. 



lower surface of the preceding whorl ; inner whorls slightly 

 depressed, and somewhat more rounded ; suture moderately im- 

 pressed ; strise of growth faint ; beneath abruptly umbilicated, dis- 

 playing the edges of all the whorls within ; aperture very oblique and 

 angular ; edge very sharp, below running forwards a little along the 

 umbilical edge of the preceding whorl, then crossing obliquely for- 

 wards and upwards, leaving a callus, it passes oif again a little be- 

 low its carinated edge. Longest diameter, one fifth of an inch ; 

 shorter diameter, three fortieths of an inch. 



It is found in most brooks, ditches, and margins of ponds, which 

 are permanent through the summer, adhering to sticks and stones. 



This shell has a striking resemblance to the P. fontanel of Eu- 

 rope (Lightfoot, "Phil. Trans." Ixxvi. pi. 2, figs. 1-4. Montagu, 

 " Test. Brit." 462, pi. 6, fig. 6. PL nitidus, MUll., Turt. &c.), ex- 

 cept that the aperture is entirely below the sharp edge, instead of 

 embracing nearly an equal portion on each side, as in that shell. 

 It is allied to P. dcjicctiis, Say ; but in that the whorls are more 

 numerous, the exterior edge much rounded, the umbilical region 

 broader and more shallow, and the labrum also embraces but half 

 of the lower surface of the preceding whorl. "Were it among the 

 land shells it would be a most unequivocal Carocolla. 



I cannot but think that the name (P. exacvous') under which 

 this shell appears in the " Journal of the Academy," is not exactly 

 as was intended by the author, as it is neither a Latin word nor a 

 Latin termination. Supposing that by a typographical error, an o 

 has taken the place of a ^, we have a legitimate term, and one very 

 expressive of the form of the shell. 



