PLANOBBIS. 397 
breadth (height) of the whole shell, although the aperture is very oblique, owing to 
its right margin deviating considerably to the left side. 
As this species somewhat resembles P. odstructus, I have broken up a specimen and 
ascertained that there are no internal teeth present. 
20. Planorbis declivis. 
Planorbis declivis, Tate, Amer. Journ. of Conch. v. p- 159 (1870) °. 
Hab. Nicaracua: marshy pools at San Agustin, near Acoyapa, in company with 
Planorbis cultratus and Ancylus excentricus (Tate 1). 
According to the description (a figure is not given), this species differs from the 
preceding, P. subpronus, in the much more depressed shell, with 4-5 whorls, the nearly 
circular aperture, and the (slightly) reflexed outer margin. 
Obs.—Planorbis declivis, Sowerby, in Reeve’s Conch. Icon. xx., Planorbis, fig. 29 
(1876) (locality unknown), belongs to a very different species; it is probably a variety 
of P. trivolvis, Say. P. declivis (Gould), Clessin, in Martini & Chemnitz, Syst. 
Conch.-Cab. ed. 2, Limneiden, p. 142, t. 17. fig. 138, from Cuba, differs also from 
Tate’s description. I cannot find any trace of a P. declivis in A. Gould’s concho- 
logical papers. 
Subgen. HIpPEvtis. 
Hippeutis, Agassiz, in Charpentier’s Moll. Terr. et Fluv. Suisse, p. 22 (1837) (part.) ; Hartmann, 
Gasteropoden d. Schweiz, p. 51 (1840). 
Drepanotrema, Fischer & Crosse, Miss. Scient. Mex., Mollusca, ii. p. 59 (1880). 
Shell particularly glossy, of small size, with few rapidly increasing whorls, more 
or less angulated at the periphery, each succeeding whorl involving a considerable 
portion of the preceding. Aperture simple, more or less rhomboidal. Distinct from 
Segmentina, Flem., by the absence of internal tooth-like thickenings. 
Known from Europe, West Africa (P. benguelensis), Brazil and Argentina 
(P. anatinus), and Eastern Asia (P. umbilicalis and P. largillierti); the two last- 
mentioned species form a connecting-link, in the general shape of the shell, with 
the Central-American forms. 
21. Planorbis yzabalensis. 
Planorbis isabel*, Sowerby, in Reeve’s Conch. Icon. xx., Planorbis, t. 12. fig. 101 (1877) °; 
Clessin, in Martini & Chemnitz, Syst. Conch.-Cab. ed. 2, Limneziden, p. 217 * . 
Pianorbis yzabalensis, Crosse & Fisch. Journ. de Conch. xxvii. p. 342 (1879) *; Miss. Scient. Mex., 
Mollusea, ii. p. 75, t. 33. figg. 2, 2a-c*. 
Very distinct by the inflated convexity of the right side and the even form of the left. 
* The word “ isabel” used by Sowerby is evidently a misspelling of Yzabal (or Izabal), this name applying 
both to the Jake (sometimes termed Golfo Dulce) and the town on its banks. 
