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, hollow. Lamarck. 



1. Saxicava ruydsa. 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 y 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 sub trigonal, equi valve, 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 sub triangular, com- 

 pressed, concentrically striated; open a little at both 

 ends ; umbones prominent ; hinge with a large triangular 

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

 ing-trough. Linnaeus. 



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

 diaphanous, radiated with pale red and white. Stult6- 

 rum, of fools. 



2. Mdctra solida. Shell sub trigonal or subelliptical, 

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



3. Mdctra elliptica.-^ Shell subelliptical, very thin, 

 subpellucid, smoothish, with the epidermis greyish-yel- 

 low. Ellipticus, oblong and equal at both ends. 



4. Mactra subtruncdta. Shell trigonal, with the 



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