SESSILIA. 171 



striated valves ; the margin at the base irregularly serrated. 

 One-fourth of an inch broad. Inhabits the British seas. 



** Operciila with four valves. 

 Genus 3.— AC AST A.— Zeac/^. 



Generic Character Shell sessile, ovate, subconic, com- 

 pressed, and consisting of six lateral valves; two of the 

 valves small, and four large, slightly united ; having a cup- 

 shaped plate, concave internally, at the base. 



Acasta Montagui. — Montagu's Acasta. Plate XIX. 

 figs. 8 and 9. Valves erect, triangular, acute, with muricated 



I ascending spines. Inhabits the British seas 



I 

 I 



1 Genus 4. — ADNA. — Leach. 



Generic Character. — Cup-shaped, sessile, shell composed 

 of one piece; aperture lozenge-shaped; with four valves. 



Adna Anglica The English Adna. Plate XIX. fig. 



28. Shell slightly ribbed; of a fine rose colour; operculum 

 transversely striated. Inhabits the Devonshire coast, at- 

 tached to Caryophyllia Anglica. 



Genus 5. — BALANUS. — Bruguiere. 



Generic Character. — Body sessile, enclosed in an oper- 

 culated shell; arms numerous, placed in two rows, unequal, 

 articulated, ciliated, each composed of two cirri, supported 

 by a peduncle, and exsertile ; mouth with four transverse 

 and dentated jaws, and provided with four hairy palpi-like 

 appendages. Shell sessile, conical, composed of six valves, 

 locked together, closed at the base by a testaceous plate ; 

 aperture subtrigonal or elliptical ; operculum internal, and 

 consisting of four moveable valves. 



Balanus candidus. — The White Balanus. Plate XIX. 

 figs. 10 and 11. White; valves nearly smooth; operculum 

 strongly ridged transversely, with longitudinal, nearly obso- 

 p2 



