434 BALANID.E. 



Coronula, the basal membrane extends only under the internal cavity 

 of the shell, and not under the folded walls, and therefore presents a 

 somewhat analogous structure. 



Opercular Valves. — The scuta and terga are nearly of the same size 

 and shape : they are mitre-formed, and higher than broad. They do 

 not fill up the orifice of the shell. The scutum is a little larger than 

 the tergum, and rather less symmetrical, the rostral corner of the valve 

 being a little produced. There is no hollow or crest for the adductor 

 muscle, which is small. In the tergum there is no trace of a spur. 

 The two valves are not articulated together, but standing close to each 

 other are united, as well as the scutum to the scutum, and tergum to 

 the tergum, by thick, brown, tough, yet soft membrane, in layers con- 

 tinuous with, but differing in appearance from, the surrounding oper- 

 cular membrane. The layers of shell, forming the valves, are thick, and 

 only the three or four lower layers are usually preserved, the upper ones 

 having symmetrically scaled off, leaving snow-white surfaces. Owing 

 to the thickness of the successive shelly layers, and to the circumstance 

 of each new layer being but very little larger than the last, the scaling 

 off of the old upper layers is a quite necessary process ; for otherwise 

 the orifice into the sack would have been encumbered and almost closed 

 by four long, slightly tapering points, prolonged upwards from the 

 basal layers that form the four existing valves. The same scaling off 

 process takes place in Platylepas, and amongst pedunculated cirri pedes 

 in Lithotrya. Microscopical examination does not exhibit any fine 

 spines on the membrane investing the valves, or any tubuli in the 

 shelly layers after their dissolution in acid : in this respect the valves 

 resemble those of Coronula. The summits of the valves project freely 

 for about a third of their own height, above the level of the membrane 

 by which they are surrounded. The orifice leading into the sack is bor- 

 dered by very protuberant lips, standing up even considerably above 

 the upper freely projecting portions of the valves. 



The Opercular Membrane, connecting the valves and the top of the 

 shell, is thick and tough, and deeply folded in concentric wrinkles. As 

 in Coronula, it consists of two or three separate membranes (each com- 

 posed of many laminae) one over the other, united to successive shelly 

 layers of the opercular valves. As the upper shelly layer scales off, 

 the membrane attached to it is likewise thrown off. The innermost 

 laminae of the last-formed opercular membrane extend down, closely 

 attached to the sheath, to its basal edge, and therefore nearly to the 

 basis of the shell ; the outer and older laminae, all closely attached one 

 within the other and to the sheath, extend to a less and less distance 

 downwards ; consequently, the animal's body is enclosed in a tube, 

 thinning out downwards, formed by the laminae of the successive oper- 

 cular membranes, surrounded outside by the shell : only in the fol- 

 lowing genus, Xenobalanus, shall we meet with a nearly analogous 

 structure. As the shell of Tubicinella increases in diameter, from the 

 growth of the radii, the opercular membrane lining the sheath is neces- 

 sarily split along the six lines of suture, in the same manner as is the 

 membrane externally investing the shell ; in a like manner, also, it is 

 repaired and added to by new longitudinal slips of membrane. Of 



