MALENTOZOA. SESSILIA. 



363 



Lepas costatus. Donov. Brit. Sh. i. PI. 30. f. 2. — Lepas costata. 

 Turt. Diet. 78. — Balauus angulosus. Lamk. Syst. v. 390 ; Ed. 2. 

 v. 6o7. 



5. Bdlanus elongdtus. Elongated Acorn-Shell. 



Shell conico-truncate, cylindrical, clavate, or diversiform, 

 with the base membranous, the pieces close or united, trans- 

 versely rugose at the base and summit, longitudinally striate 

 or costate ; the aperture large, subrhomboidal ; the interior 

 transversely striate ; the posterior opercular valves triangular, 

 oblique, transversely striato-rugose, acute or obtuse ; the dorsal 

 erect, triangular-oblong, obtuse, also transversely striate, articu- 

 lated with the others by a groove, and having near the end a 

 sinus and a protuberance locking into corresponding parts of 

 the others. 



This species, which adheres to rocks, stones, wood, shells, 

 and Crustacea, exhibits great diversity of form. In its early 

 stage, it is elliptical, membranaceous, continuous, of a brownish 

 colour, with an elliptical convex longitudinally slit operculum, 

 and presents the form of an oval dish with its cover. Presently, 

 by the deposition of calcareous matter within the membranes, 

 the valves of the coronary body and operculum become dis- 

 tinguishable. As it enlarges, the shell spreads at the base, 

 and assumes a conical form, with the six pieces more or less 

 distinct. When the individuals are crowded, they elongate in 

 various degrees, and young individuals adhering to the upper 

 parts, clusters are formed having a proliferous appearance. 

 Sometimes they are simply conical, or cylindrical, or they en- 

 large from the base upwards, or are clavate, or cylindrical 

 below and ovate or campanulate above. The shell is some- 

 times thick, frequently very thin and fragile ; the colour grey- 

 ish-white, yellowish-white, or pure white. Generally the 

 pieces are united, so as to be indistinguishable, unless at the 

 top, where they run out in a pointed form, but are soon 

 abraded. The opercular pieces ultimately become smooth, or 

 present few striae. The walls being thin at the base, and the 

 attachment membranous, this species is easily detached, and 

 even readily crushed by the fingers. 



The following are the principal varieties of form : — 

 A. Bdlanus elongdtus rugosus. 



Shell conico-cylindrical, thin ; the valves united at the 

 lower part, without any indication of separation ; the opercular 

 valves striated, rather acute. 



