MALACOZOA. TROPIOPODA. LAMELLIBRAXCIITATA. 217 



Gexus 3. Saxicava. — Shell oblong, veiy inequilateral, 

 convex, covered with epidermis ; umbones prominent ; 

 hinge with small divergent teeth ; ligament external. 

 Saxum, a stone ; cavus^ hoUow. Lamarck. 



1. Saxicava rugosa. — 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 hne from the 

 umbo to the angle. Rugusus, wrinkled. 



2. Saxicava drctica. — ShcU 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 spiuiferous ridges on each 

 side. Arcticus, northern. 



Family X. — Mactrixa. 



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 obhque triangular depression for the internal 

 ligament ; a small external ligament also. Name from 

 the ^enus 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 shdtorum. — Shell o vat o-trigonal, thin, sub- 

 diaphanous, radiated with pale red and white. StuJto- 

 rum, of fools. 



2. Mactra solid a. — Shell subtrigonal or subelliptical, 

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



3. Mactra elliptica. — Shell subelhptical, veiy thin, 

 subpeUucid, smoothish, with the epidermis greyish-yel- 

 low. ElUpticus, oblong and equal at both ends. 



4. Mactra suhtruncdta. — Shell trigonal, with the 



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