MALACOZOA. TROPIOPODA. LAMELLIBRANCHIATA. 217 



GENUS 3. SAXICAVA. Shell oblong, very inequilateral, 

 convex, covered with epidermis ; umbones prominent ; 

 hinge with small divergent teeth; ligament external. 

 Saxum, a stone ; cavus, hoUow. Lamarck. 



1. Saxicava ruy6sa. Shell oblong, elliptical, or ob- 

 ovate, regular or variously distorted, rather thick, 

 coarsely rugose ; the anterior end short and rounded, 

 the posterior prolonged, with a prominent line from the 

 umbo to the angle. Rugosus, wrinkled. 



2. Saxicava drctica. Shell oblong or linear-oblong, 

 regular or distorted, thin, coarsely wrinkled ; the an- 

 terior end very short, angulate, the posterior abrupt or 

 rounded, with two prominent spiniferous ridges on each 

 side. Arcticus, northern. 



FAMILY X. MACTRINA. 



Animal oval or elliptical, compressed, with the mantle 

 closed, but open in front for the foot, and forming behind 

 a tube internally double ; the foot large, compressed, 

 bent ; shell roundish, oval, or subtrigonal, equivalve, in- 

 equilateral, more or less gaping at both ends ; the hinge 

 with an oblique triangular depression for the internal 

 ligament ; a small external ligament also. Name from 

 the genus Mactra. 



GENUS 1. MACTRA. Shell oval or subtriangular, com- 

 pressed, concentrically striated; open a little at both 

 ends ; umbones prominent ; hinge with a large triangular 

 fossa, and angularly bent thin tooth. Mactra, a knead- 

 ing-trough. Linnaeus. 



1 . Mactra stultdrum. Shell ovato-trigonal, thin, sub- 

 diaphanous, radiated with pale red and white. Stult6- 

 rum, of fools. 



2. Mactra solida. Shell subtrigonal or subelliptical, 

 very thick, subantiquated, white. Solidus, solid or thick. 



3. Mactra elltptica. Shell subelliptical, very thin, 

 subpellucid, smoothish, with the epidermis greyish-yel- 

 low. Eltipticus, oblong and equal at both ends. 



4. Mactra subtruncdta. Shell trigonal, with the 



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