MALACOZOA. TROPIOPODA. LAMELLIBRAXCIIIATA. 207 



and posteriorly radiatingly striate, smooth in the middle. 

 Discors, discordant, or with diversiform markings. 



Genus 3. Pixxa. — Shell elongated, obovato-triangular, 

 equivalve, inequilateral, moderately convex, compressed 

 behind, thin, horny, semitransparent, fragile. Pinna, 

 a sea-fan. Linnaeus. 



1 . Pinna ingens. — Shell oblongo-triangular, moderately 

 convex, with the dorsal line nearly straight, the end 

 rounded, the valves thin, brittle, yellowish-brown or 

 dusky, with about ten longitudinal narrow, obtuse ridges 

 on the half next the ligament, disappearing beyond the 

 middle. Ingens, very large. 



Family III. — Uxioxina. 



Animal oblong, elliptical, or roundish, with the mantle 

 open beneath, coherent behind, forming two orifices, the 

 lower barbate ; the foot large, compressed, tapering ; 

 both adductor muscles large. Shell regular, equivalve, 

 inequilateral, with a strong epidermis ; the ligament 

 linear and dorsal; the anterior muscular impression large. 



Gexus 1. Alasmodon. — Shell equivalve, very inequi- 

 lateral, compressed, concentrically rugose ; umbones 

 small, incurved, carious ; hinge with an irregular pro- 

 minent crenulated tooth in one valve, two in the other. 



1. Alasmodon mar garitif eras. — Shell ovato-oblong, 

 rather compressed, thick, rugose, becoming curved when 

 old. Margarita, a pearl ; fero, to bear. 



Gexus 2. Axodox. — Shell equivalve, very inequilateral, 

 convex, concentrically rugose, thin ; umbones small ; 

 hinge toothless, but with a lamina under the ligament. 

 'A, without ; ddovs, a tooth. Lamarck. 



1. Anodon anatinns. — Shell ovate, moderately con- 

 vex, thin, rugose, the anterior end short and rounded, 

 the posterior truncato-angulate. Anatinns, from anas, 

 a duck. 



Family IV. — Arcina. 



Animal oblong, or roundish, with the mantle open in 

 its whole length, and destitute of tubes ; the foot very 



