INTRODUCTION. 



41 



compose the order Sessile Cirripedes of Lamarck ; 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 basal support is sometimes thick and flat, sometimes 

 forming an 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 lie flat in two pairs against each other. Through the 

 ventral pair the cirrhi 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 cirrhi; 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 single rounded 

 piece. 



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





