566 MOLLUSCA. 
Hab. W. Mexico: Acapulco (Belcher ®). 
S. Panama: Panama, under a heap of stones at the bottom of the sea-wall, at high- 
water mark of spring-tides, copiously (C. B. Adams !~*). 
Named in honour of the herpetologist, F. 8. Baird. 
3. Truncatella bilabiata. 
Truncatella bilabiata, Pfeiffer, Archiv fiir Naturg. 1840, p. 253°; Monogr. Auric. p. 192°; Kiister, 
in Martini & Chemnitz, Syst. Conch.-Cab. ed. 2, Truncatella, p. 7, t. 1. figg. 27-29 (full- 
grown), 30, 81 (young) *; Binney, Land and Freshw. Shells of N. Am. iii. p. 99, fig. 199+; 
Tryon, Am. Journ. Conch. iv. p. 14, t. 18. figg. 82, 33°. 
Hab. Yucatan: Carmen Island (Morelet 2-5). 
Also in Cuba (Pfeiffer! 2) and Florida (Binney +5). 
According to Pfeiffer ?, Prof. Bronn, of Heidelberg, possesses examples of this 
species from Carmen Island, probably sent to him, with many others, by Morelet. 
Fam. CERITHIIDZ. 
One genus of this family is essentially submarine, living mostly in mangrove- 
swamps. 
POTAMIDES. 
Potamides, Brongniart, Ann. du Mus. d’Hist. Nat. Paris, xv. p. 867 (1810); P. Fischer, Manuel 
de Conch. p. 680. 
Shell turrited, many-whorled, with knobbed or ribbed sculpture, and a dark brown 
or grey periostracum, in many species provided on some whorls with thickenings, which 
are the remains of a former aperture (varices) ; aperture comparatively small, vertically 
placed, rounded, with a notch or short channel below. Operculum horny, orbicular, 
many-whorled. 
Living animal with a produced snout and two slender, pointed feelers, the eyes on 
protuberances at the outer side of the feelers near the base; foot short, somewhat 
square, the anterior angles produced. Lives chiefly in brackish water, at the roots and 
on the stems of mangrove shrubs, to which it can affix itself by slimy threads. 
Differs from the well-known genus Cerithium, Brug., by the many-whorled 
operculum, and by its habit of dwelling in brackish water, to which the mud-coloured, 
rather thick periostracum is adapted. If the shell is somewhat worn or bleached, its 
colour becomes brighter, either violet or chestnut-brown. 
A. Adams (Zool. Voy. ‘Samarang,’ Moll. p. 44) says that in some species of the 
subgenus Cerithidea, from the Indian seas, the feelers are very short, and bear the 
eyes at their distal end, ¢.¢. the slender portion of the feeler beyond the eyes is 
wanting ; and on this character a separate genus has been proposed, named Phenommia 
