TUBICINELLA TRACHEALIS. 435 



this structure, in the opercular membrane, I have seen no other in- 

 stance ; for in most genera the old opercular membrane is moulted, 

 and a new and larger one formed at each period of growth ; in Coro- 

 nula, in which the opercular membrane is likewise for a time persistent, 

 it does not run far down the inside of the shell, and each new mem- 

 brane is formed large and extensible, so as to allow, without splitting, 

 of some increase in the diameter of the shell. The opercular mem- 

 brane at the summit of the shell, in Tubicinella, is folded in concentric 

 lines, and so deeply, that the basal edges of the opercular valves are 

 generally hidden : this folding arises partly from each last deposited 

 and innermost membrane being originally formed slightly folded, but 

 chiefly from the rapid downward growth of the shell, and the conse- 

 quent downward movement of the whole animal's body, together with 

 the opercular valves to which the body is attached, and this necessarily 

 tends to wrinkle and fold the opercular membrane. Owing to the oper- 

 cular membrane extending far down inside the shell, and being firmly 

 attached to the sheath, as the upper part of the shell breaks away and 

 disintegrates (which we shall presently see is constantly taking place), 

 small particles of shell are left adherent to the circumferential and 

 folded parts of the opercular membrane ; and this at first much per- 

 plexed me. 



Muscles of the Sack ; these extend down almost to the base of the 

 shell, but in the lower part they spread out and become thin and very 

 irregular, not even corresponding on the opposite sides of the body. 

 The fasciae in the upper part show very distinct transverse striae, but 

 lower down these become either obscure or entirely deficient. In all 

 these characters the muscles of Tubicinella and Coronula resemble 

 each other. The rostral depressor muscles of the scuta consist each 

 of four small bundles of fasciae ; the lateral depressores run not quite 

 straight down, but in a curved course towards the carinal end of the 

 sack : the tergal depressores are proportionally smaller than in ordi- 

 nary sessile cirripedes, but they project and form two crests (with 

 some fasciae between them), which support the Branchiae. The mem- 

 brane lining the sack, I may here mention, is unusually strong. 



Branchice* — These are enormously developed; the two together 

 covering two thirds of the area of the sack. Each consists of two folds, 

 both deeply plicated. They are attached longitudinally to the two crests, 

 including and formed by the muscles running from the terga to near 

 the basis of the shell. The branchiae are likewise attached transversely 

 to the sack, under the basal margins of the terga. 



Mouth. — The labrum is very finely hirsute, without teeth ; the palpi 

 have a short row of moderately long spines along their exterior basal 

 margins. The mandibles have four rather narrow, sharp teeth, which 

 (excepting the first) have double points: between the second and third, 

 and again between the third and fourth teeth, there is a single small 

 intermediate tooth : the inferior angle is irregularly pectinated. The 



* These have been described by Professor Owen in the second volume of the 

 'Descriptive Catalogue of the Museum of the Royal College of Surgeons.' 



