POTAMIDES. 567 
by Mérch [Malak. Blatt. vii. p. 80 (1860)], and Aphanistylas by P. Fischer [Manuel de 
Conchyliologie, p. 682]. In the species which I have observed alive at Singapore the 
feelers had this appearance when the animal was crawling above the surface of the 
yen, but when below it the upper part of the feeler was distinctly present; on closer 
examination, however, it was seen that, out of the water, the upper portion of the feeler 
was bent back and closely applied to the head, so as to be scarcely visible. I suppose 
that this was also the case in the specimens observed by A. Adams. 
Two subgenera are represented in Central America, about one of which there is a 
good deal of confusion. 
Subgen. 1. CrrirHipEA, Swains. 
Shell beset with vertical, closely-placed ribs, which are often somewhat arcuated, and 
in some species latticed (cancellated) by the crossing of spiral ridges. Outer margin 
of the aperture in most of the species much thickened: if such thickenings are 
already formed in earlier stages of growth they remain as varices in the full-grown 
shell, usually one on each of the last three or four whorls, but not at regularly equal 
distances. Aperture scarcely notched below. 
In the species of this subgenus from the tropical regions of the Old World the upper 
whorls are lost very regularly in the full-grown specimens, so that the shell is broadly 
truncated above and consists of 4-6 whorls only; in the examples from the Atlantic 
and Pacific shores of America this is not the case, the upper end of the shell being 
usually pointed, or only a little worn, not more than one or two whorls being lost. In 
several of the American and other forms the ribs become comparatively more numerous 
on the last whorl, but also more feeble and unequal, than on the preceding whorls; it 
is safer, therefore, to count the ribs of the penultimate whorl when giving the 
differential characters of the species. Most of them have a pale spiral band on the 
middle of each whorl: this is in some specimens more distinct and apparent, in others 
not so well defined or even absent; it is usually more conspicuous in shells which are 
somewhat worn, and of a chestnut-brown or violet colour, than in the fresh, mud- 
coloured state. I have therefore not made use of this character in the Table of 
the species. 
Subgen. 2. RHINOCORYNE, n. 
Potamides pacificus (Sow.) differs from all the other submarine species of Cerithiide 
in having a projecting channel at the base of the aperture, well separated by a distinct 
constriction from the hole of the aperture, and running obliquely downward and 
backward. 
It has been placed by H. & A. Adams in the genus Potamides, Brongn., together 
with the Australian P. ebeninus (Gen. Recent Moll. i. p. 290), and they were followed in 
this respect by Reeve and others. Brongniart, however, mentions neither of these forms, 
but describes and figures only a new fossil one, P. lamarcki (nec Cerithium lamarcki, Val., 
