MALACOZOA. TROPIOI'ODA. LAMELLIBRANCIIIATA. 207 



and posteriorly radiatingly striate, smooth in the middle. 

 Discors, discordant, or with diversiform markings. 



GENUS 3. PINNA. 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, UNIONINA. 



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. 



GENUS 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. Aldsmodon margaritiferus. Shell ovato-oblong, 

 rather compressed, thick, rugose, becoming curved when 

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



GENUS 2. ANODON. Shell equivalve, very inequilateral, 

 convex, concentrically rugose, thin; umbones small; 

 hinge toothless, but with a lamina under the ligament. 

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



1. Anodon anatinus. Shell ovate, moderately con- 

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

 the posterior truncato-angulate. Anatinus, 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 



