76 Lamarck’s Genera of’ Shells. 
cannot stand on its base, which is externally convex, sometimes 
conoidal. 
Type. Acasta Montagui*. 
Shell with acute valves, transversely striated, externally mu- 
ricated, with small erect spines. 3 Species. All found in 
sponges. Pl. iii, Fig. 12. 
5. Creusia. 
Body sessile, subglobular, enclosed in an operculated shell. 
Several tentacula. Mouth not prominent, situated at the upper 
anterior part of the body. 
Shell sessile, fixed, orbicular, conically convex, composed of 
four unequal, united valves, with distinct sutures. Operculum 
interior, bivalve. Shell generally very small, attached to ma- 
drepores, and other marine substances. 
Type. Creusia Stromia. 
Shell conico-convex; valves with radiated furrows ; two ser- 
rated sutures. North Seas. 3 Species. 
6. Pyrgoma. 
Animal...... 
Shell sessile, univalye subglobular, ventricose, convex at the 
top, apex perforated. Aperture small, elliptical. Operculum 
bivalve. 
1 Species. Pyrgoma cancellatat. Incased in a stony poly- 
parium of the genus Astrea. Red Sea? Pl. iii, Fig. 13. 
Second Order. 
PEDUNCULATED CIRRIPEDA. 
The body supported by a moveable, coriaceous, tubular 
pedicle, fixed on marine substances. The mouth nearly on the 
under side. 
This order contains 4 genera. 
1. Anatifaf. 
Body covered with a shell, and supported by a tubular, tendi- 
nous pedicle. Numerous long, unequal, articulated, fringed 
tentacula, projecting on one side, below the summit. 
* Montague's. + Cuncellated— lattice-work. ¢ From unas, a duck, 
