218 MALACOZOA. TROPIOPODA. LAMELLIBRAXCIIIATA. 



posterior end more produced and angulate, thin, striated, 

 with the epidermis greyish-yellow. Subtruncatus, some- 

 what cut short. 



Genus 2. Goodallia. — Shell trigonal, moderately con- 

 vex, closed at both ends ; umbones prominent ; hinge 

 with two teeth and an intervening triangular cavity in 

 one valve, a single tooth in the other. Named by Dr. 

 Turton, after Dr. Goodall, Provost of Eton. 



1. Goodallia triangularis. — Shell trigonal, with the 

 margin internally denticulate. Triangularis, three- 

 cornered. 



2. Goodallia minutissima. — Shell trigonal, with the 

 margin internally even. Named from its small size. 



Genus 3. Lutraria. — Shell roundish or oval, inequi- 

 valve, much compressed, thin, concentrically striated, 

 open at both ends ; umbones small ; hinge with a large 

 triangular fossa, and small angularly bent tooth. Lutum, 

 mud. Lamarck. 



1. Lutraria elliptica. — Shell elliptical, compressed, 

 thin, with a yellowish-grey or dusky epidermis. Ellipti- 

 cus, of the form of an ellipse. 



2. Lutrdria Listeri. — Shell roundish-triangular, much 

 compressed, thin, dull white. Named after Dr. Lister, 

 an English naturalist. 



Genus 4. Amphidesma. — Shell oval or oblong, com- 

 pressed, equivalve, inequilateral, very thin, glossy, with 

 a delicate epidermis ; umbones very small ; hinge with 

 an oblique concave prominence in each valve, for the 

 internal ligament, the external short. 'A/a^Z, about ; 

 Seo-fia, a band. Lamarck. 



1 . Amphidesma album. — Shell ovate, very inequilateral, 

 pellucid, white. Alius, white. 



2. Amphidesma tenue. — Shell ovato-triangular, nearly 

 equilateral, pellucid, white. Tenuis, thin. 



3. Amphidesma prismdticum. — Shell ovato-oblong, 

 very inequilateral, very delicate, transparent, hyaline- 

 white. Named from its iridescence, or reflection of 

 colours like those seen in a glass prism. 



