150 SESSILTA. 



Analifa lavis. — The Smooth Anatifa. Plate XIX. fig. 5. 

 With five smooth valves ; the dorsal valve rounded at the sides, 

 and slightly carinated •, peduncle very long, of a scarlet colour. 

 Inhabits the British seas. 



ORDER II—SESSILIA. 



Body without a peduncle, and inclosed in a multivalve shell, 

 seated immediately on marine bodies, or rocks ; mouth and ten- 

 tacula placed in the apex, or upper part of the body. 



* With a bivalve operculum. 



Genus l.—'PY^GOMA.—Savigney. 



Generic Character. — Seated univalve, globularly ventricose, 

 convex above, and open at the apex ; opening small, elliptical, 

 provided with a bivalve operculum. 



Pyrgoma crcnata. — The Crenated Pyrgoma. Plate XIX. 

 fig. 6. With radiated ribs, and crenated round the margins ; of 

 a pale violet colour. 



Genus 2. — CREUSJ A.— Leach. 



Generic Character. — Body sitting, subglobular, inclosed in an 

 operculated shell ; having three or four pairs of tentacular arms. 

 Shell sessile, fixed, orbicular, convexo-conical, consisting of 

 four united unequal valves ; operculum interior and bivalve. 



Creusia verruca. — The Warted Creusia. Plate XIX. fig. 

 7. White, slightly depressed, with interwoven obliquely striated 

 valves ; the margin at the base irregularly serrated. One-fourth 

 of an inch broad. Inhabits the British seas. 



** Opercula with four valves. 



Genus 3.-^AC AST A.— Leach. 



Generic Character, — Shell sessile, ovate, subconic, compress- 

 ed, and consisting of six parts, two of the valves small, and four 



