PYBGOMA. CIRRIPEDA. 125 



ORDER II. SESSILIA. 



Body destitute of peduncle, and inclosed in a shell fixed imme- 

 diately on marine bodies ; mouth at the upper and anterior 

 part of the body. 



The shell of this order is never compressed on the sides, and appears in general in 

 one single piece of a conical form, or of a tube truncated at the summit. In the inte- 

 rior of the opening is an operculum formed of two or four moveable pieces, which 

 the animal opens when it projects its tentacular arms. The shell, solid and calca 

 reous, is always fixed on some other body without the power of changing its place. 



* Operculum bivalve. 

 Gen. 1. PYRGOMA, Sav. 



Shell sessile, univalve, subglobular, ventricose, convex above, 

 and open at the summit ; opening small, elliptical ; oper- 

 culum bivalve. 



P. cancellata, Leach. Shell with elevated radiating ribs ; interstices 

 cancellated ; cancelli and ribs frosted Supp. Encyc. Brit. iii. pi. 57 



Gen. 2. CREUSIA, Leach. 



Body sessile, subglobular, inclosed in an operculated shell ; three 

 or four pairs of tentacular arms ; shell sessile, fixed, orbicu- 

 lar, convex-conical, of four united valves, the valves unequal ; 

 operculum interior, bivalve. 



C. spmulosa. Leach. Shell turbinated, convex, with radiating ele- 

 vated ribs ; ribs spinulose ; operculum obliquely pyramidal. In- 

 habits Indian seas, on madrepores. Supp. Encyc. Brit. iii. pi. 57- 



C. verruca, Lam. Shell depressed, obliquely striated ; aperture sub- 

 quadrate. Inhabits Northern seas. Chem. viii. pi. 98, fig. 834. 



** Operculum quadrivalve. 



Gen. 3. ACASTA, Leach. 



Shell sessile, oval, subconical, compressed, sexpartite; two valves 

 small, four large, slightly united, with an orbicular plate, con- 

 cave internally, at the base. 



A. Montagui, Leach. Shell with the valves acute, transversely 

 striated, and muricated with ascending spines. Inhabits British 

 coasts Supp. Encyc. Brit. iii. pi. 57 



Gen. 4. BALANUS, Lam. 



Body sessile, inclosed in an operculated shell ; arms numerous, 

 in two rows, unequal, articulated, ciliated, composed each of 

 two cirri supported on a peduncle, and exsertile ; mouth with 

 four transverse dentated jaws, and four hairy appendages re- 

 sembling palpi. Shell sessile, univalve, conical, shut at the 

 base by a testaceous plate; aperture subtrigonal or elliptical; 

 operculum quadrivalve. 



The shell of the Bulani is immoveable in all its external parts. It is of a conical 

 shape, sometimes elongated, and fixed to rocks, stones, or marine bodies. The 



