40S BALANIDJE. 



with the opercular membrane. The opercular membrane is 

 very thick, tough, and yellowish; it is, in parts, finely plicated 

 in lines radiating from the apices of the valves: these plicae 

 consist of membrane in an altered condition, being harder, 

 more horny, and of a browner yellow ; the plicae are large at 

 the rostral end of the scuta, and projecting beyond these 

 valves, they afford attachment to the rostral depressor mus- 

 cles. Rims of similarly modified membrane (PI. 15, fig. 2 b) 

 connect the scuta and terga together. The rims and plicae are 

 occasionally moulted together with the opercular membrane. 

 In almost all hitherto described Balanidac, a new oper- 

 cular membrane is formed at each period of exuviation, 

 and as soon as formed, the old one is generally moulted, 

 together with the other membranes of the body : the 

 case is very different in Coronula, in which it is evident, 

 from the lines of growth on the valves and sheath, that 

 a new membrane is formed only at rather long intervals, 

 and that it is formed in some degree extensible, so as 

 to allow of some growth in the shell. Two or even three of 

 these membranes are retained at the same time, one over 

 the other ; and thus, by their joint thickness, they afford 

 protection to the included animal's body, and compensate 

 for the smallness of the opercular valves. In a large specimen 

 of C. balcenaris, two inches in diameter, there had been 

 formed, since its existence as a very young shell, not more 

 than eight opercular membranes, whereas the other mem- 

 branes must have been moulted within this same period at 

 least thirty times. In a young specimen of this species, 

 having the orifice of the shell only two tenths of an inch in 

 diameter, I found the opercular membrane, as usual, double. 

 This membrane is not furnished with spines, nor is it pene- 

 trated by tubuli as in most other genera. The tissue left 

 after the opercular valves have been dissolved in acid, pre- 

 sents no tubuli, or any other recognisable character, and is 

 not divided (as is usual) into layers. 



Muscles of the Sack. — These muscles differ considerably 

 from those of other sessile cirripedes hitherto described. 

 There are the usual five (or properly six, the two tergal 

 muscles being here, as elsewhere, confluent) bundles of 

 fasciae: but they hardly can be called bundles, they are so 



