PACHYCHILUS. 437 
PACHYCHILUS. 
Pachychilus, Lea, P. Z. 8. 1850, p.179; Ann. & Mag. Nat. Hist. (2) ix. p. 58 (1852) ; Troschel, 
Gebiss. d. Schnecken, i. p. 114 (part.); Fischer & Crosse, Miss. Scient. Mex., Moll. ii. 
p- 320 (nec Pachychila, Eschscholtz, 1831). 
Shell turrite or at least conical, longer than broad, with fine spiral striz, and covered 
by a distinct greenish, brownish, or blackish periostracum, sometimes tuberculated : 
aperture oblong, angulated above, produced more or less distinctly in a short angular 
lobe below (except in the subgenus Potamanax) ; columellar margin not notched or 
truncate, often thickened. 
Operculum horny, of about six whorls; its nucleus subcentral in about one-third of 
its height. Radula: central tooth with an elevated swelling in the centre of its 
plane; lateral teeth bi- or tricuspid. Edges of the mantle simple, uninterrupted. 
Eyes at the outside of the base of the feelers. 
The living animals of this genus prefer running water, rivers, or small streams, and 
are often found attached to stones; the upper whorls of full-grown shells are never 
complete and usually more or less eroded, so that they are devoid of the periostracum, 
uneven, and of a greyish or pure white colour. ‘This erosion seems to be caused partly 
by the rubbing of the shell against the stones, and partly by chemical action, due 
probably to the attacks of microscopic alge. M. Gassies has observed in one species 
(P. liebmanni, var. gassiesi) that the erosion commences with several small, circular, 
isolated spots. It is also possible that one individual gnaws at the shell of another in 
order to obtain calcareous matter for its own growth, as has been observed in Jand- 
shells by Rossmiissler. The upper whorls seem to become gradually filled inside with 
calcareous matter, so that, in spite of extensive erosion, they form a solid conical 
prominence in some species, ¢. g. in P. turatii. On account of this loss of the upper 
whorls it is very often impossible to do more than give the number of those remaining 
(‘‘anfractus superstites”); and, for the same reason, the relative proportion of the 
diameter of the shell to its length is of no great value for the discrimination of the 
species, being variable in different individuals according to the extent of erosion. The 
relative proportion of the diameter of the whole shell (last whorl in its | greatest 
breadth) to the length of the aperture is, however, of some importance, as it indicates 
a real difference in the conformation of the shell. . 
The species of the subgenus Oxymelania are the least subject to this erosion. 
The number of remaining whorls in the adult state is, of course, somewhat variable, 
but nevertheless I shall mention it in the Comparative ‘Table, as it is of some use in 
distinguishing the various forms. 
As all the Pachychili show faint spiral strie in their sculpture, I have not mentioned 
this in the Table, using the term “ levis” for all the parts which are without distinctly 
elevated spiral lines (lire); the broader elevations running from above to below, from 
