294 MALACOZOA. TROPIOPODA. LAMELLIBRANCHIATA. 



3. Amphidesma prismaticum. Iridescent Amphidesma. 



Shell ovato-oblong, much compressed, pellucid, very inequi- 

 lateral, anteriorly prolonged and rounded, posteriorly narrowed, 

 angular, somewhat gaping, and laterally bent ; delicately stri- 

 ated concentrically, with the surface very minutely shagreened 

 and faintly iridescent; the inner surface highly glossed and 

 iridescent, smooth toward the margin, elsewhere granulated ; 

 in the right valve a thick concave adherent tooth, behind the 

 narrow infraumbonal sinus, and a small, generally bifid pro- 

 minence before it, the anterior and posterior margins with a 

 deep groove ; in the left valve a corresponding concave tooth, 

 with a small anterior sinus, the margins plain. Length an 

 inch, height half an inch. 



Common in deep water, off Aberdeen, and frequently brought 

 up by the lines, adhering to Actiniae. Plentiful also occasion- 

 ally on the beach. 



Ligula prismatica. Laskey, Wern. Mem. i. 377, — Ligula pris- 

 matica. Mont. Test. Brit. Suppl. 23. PI. 26. f. 3. — Amphidesma 

 prismatica. Lamk. Syst. v. 492; Ed. n. vi. 128. — Amphidesma 

 prismaticum. Turt. Brit. Biv. 52. PI. 5. f. 3. — Amphidesma pris- 

 maticum. Flem. Biit. Aiaim. 432. 



Genus 5. Anatina. 



Shell oval, inequivalve, inequilateral, thin, brittle, 

 gaping, and more or less truncate at the posterior end. 

 Umbones small, directed a little backwards. Hinge with 

 an abrupt sinus and a concave spoon-shaped lamina pro- 

 jecting vertically downwards, in each valve ; internal 

 ligament received into the spoon-shaped cavities ; ex- 

 ternal very small. Muscular impressions small, sub- 

 marginal. 



This genus is so very nearly allied to the last that 

 there is scarcely sufficient reason for separating them. 



1. Anatina prat eniris. Delicate Anatina. 



Shell ovate, rather compressed, very thin, brittle, pellucid, 

 with the anterior end larger and rounded, the posterior sub- 

 truncato-rotundate, the anterior dorsal outline convex, the 

 posterior concave ; concentrically fain'Jy striate ; the surface 

 slightly glossy, very minutely punctulate, at the posterior end 

 roughish with minute prominences ; an obtuse ridge from the 



