115 



Shell; triangular or oblong ovate, very thick and solid, covered 

 with a glabrous horny epidermis ; hinge composed of two to 

 three triangularly disposed cardinal teeth in each valve, ivhich 

 are sometimes indistinct or 'partially obsolete. 



The freshwater Conchacea begin with Galathcea, a siphoned bivalve of 

 comparatively large size, whose habitat is restricted to the Nile and the 

 rivers of Western Africa. The shell is quite unique in its composition, 

 being of a peculiar marble-like substance, more or less delicately rayed or 

 tinted with rose-purple or blue ; and it is covered with a strong epidermis, 

 not of a fibrous texture like that of Ci/prina, but hard, glabrous, and horny, 

 of a rich olive or olive-green colour. Twelve species have been described. 



1. Bengoensis, Dhr. 



2. Cailliaudi, Bern. 



3. concamerata, Duv. 



4. Cuiningii, Dkr.. 



Species. 



5. Heukelomi, Bern. 



6. Kochii, id. 



7. laeta, Phil. 



8. Lubakii, Bern. 



9. paradoxa, Born. 



10. mbicunda, Phil. 



11. tenuicula, Phil. 



12. versicolor, Morel. 



Figure. 



GALATHiEA paradoxa. PI. 30. Fig. 197. A very characteristic illustra- 

 tion of the original species, showing its dense hinge structure. 



Genus 9. CYRENA, Lamarck. 



Animal ; mantle-lobes united in their posterior third; siphons se- 

 parated to their base. fDeshayes.J 



Shell ; mostly solid, sometimes concentrically ridged, covered with 

 an olive or blackish epidermis ; hinge composed of three cardi- 

 nal teeth in one valve and two in the other, and two, often con- 

 spicuous, lateral teeth in each valve. 



A characteristic and somewhat variable series of freshwater bivalves, 

 some of which are of large size, richly violet-stained in the interior, some 

 roughly formed and more or less eroded, some strongly ploughed across 

 with ridges and furrows. The animal partakes of the characters of Venus 

 and Cyclas, simulating the latter genus more particularly in having the 



