360 MALENTOZOA. SESSILU. 



Shell more or less cylindrical, or conical, solid, inter- 

 nally porous, adhering by its base, and composed of one 

 or of several pieces, united by their sides, and either 

 open at the base, or closed by a membranous or cal- 

 careous piece, by which it adheres ; always open at the 

 summit, but there having a pyramidal operculum of two 

 or four valves, analogous to the shell of the Lepadina, 

 and opening so as to allow the cirri to protrude. 



These animals are always affixed to rocks, stones, 

 shells, Crustacea, wood, or other objects, and often so 

 crowded as to alter the normal forms of each other, so 

 that a species may present itself under very different 

 aspects, a circumstance which has given rise to an un- 

 due multiplication of species by authors. The genera 

 are founded upon the peculiarities of the operculum, the 

 coronary part or calcareous tube, and the nature of the 

 support. 



Genus 1. Balanus. Acorn-Shell. 



Animal subcorneal or cylindrical ; with the mantle 

 open above and before only ; the branchiae in the form 

 of two fringed laminae attached to the inner surface of 

 the mantle. 



Shell conical, formed of six pieces, a ventral, a dorsal, 

 and two pairs of lateral ; operculum in the form of a 

 somewhat oblique pyramid, of four triangular pieces, 

 two smaller than the rest. 



The species adhere to rocks, stones, wood, shells, and 

 other bodies crowded together. 



1. Balanus halanoides. Smooth Acorn-Shell. 



Shell conical, truncate, with the six valves distinctly sepa- 

 rated by narrow grooves, which are wider at the upper part ; 

 the ventral valve generally largest, the ventro-lateral nearly 

 equal, the dorso-lateral very small, the dorsal about half the 

 size of the ventral ; all nearly smooth, being but faintly striu- 

 late longitudinally and transversely, but often rugose or cre- 

 nato-sulcate at the base ; ventral opercular valves triangular, 

 acute, transversely striato- rugose, with a groove and project- 

 ing thin lamina on their articular margin ; dorsal valves tri- 



