INTRODUCTION, 



41 



compose the order Sessile Cirripedes of Lamark; they consist 

 of a number of valves placed against each other side by side 

 in a circle, supported on a plate, or tube, or cup, and closed 

 by an operculum composed of two or more valves. 



The hasal support is sometimes thick and flat, sometimes 

 forming ah elongated tube, and sometimes hollowed out into 

 a cup. In other species it is altogether wanting. The oper- 

 culum always consists of more than one piece, generally of 

 two pairs : they are either articulated to each other by serrated 

 edges, and placed against each other conically, as in Balanus, 

 or they he flat in two pairs against each other. Through the 

 ventral pair the cirri protrude. 



The parietal valves, composing the principal part of the 

 shell, vary in number, form and position. The anterior valves 

 are placed on the same side with the cirri; the posterior, 

 those on the opposite side ; and those which remain between 

 on each side are the lateral valves. In many cases, parti- 

 cularly in Balanus, each valve is separated into the "prominent 

 and depressed areas, and the inserted lamina. In some 

 instances, the parietal portion is formed by a siagle rounded 

 piece. 



In the accompanying cut (87), the prominent areas are dis- 



