136 



Fifteen species have been described, but the genus was not examined by 

 Mr. Sowerby when figuring Venerupis and Petricola in his ' Thesaurus/ 

 and needs to be monographed. Two species are British, the Linnean 

 S. rugosa and Arctica {Hiatella, Turton) ; others are Californian, Peru- 

 vian, and Australian. 



1. Arctica, Linn. 



2. austral is, Lam. 



3. Californica, Conr. 



4. Carditoidcs, id. 



5. distorta, Say. 



Species. 



6. Guerinii, Desk. 



7. legumen, id. 



8. Pholadis, Lam. 



9. purpurascens, Sow. 



10. rugosa, Lam. 



11. solida, Sow. 



12. tenuis, id. 



13. Veneriformis, Lam. 



Saxicava Arctica. 

 growth. 



Figure. 



PI. 39. Pig. 



214. Shell of a peculiar rhomboid 



Family 9. MACTRACEA. 



Shell ; mostly inequilateral, sometimes a little gaping at the sides ; 

 ligament partially or wholly internal. 



After assorting into families the forms of bivalves represented on one 

 side by Tellina and on the other by Mya, there remain a group of genera 

 partaking variously of both types, but yet distinguished in having the liga- 

 ment wholly or partially internal : these are the Mactracece. In the true 

 Mactra we have a form which differs in a manner clearly enough removed 

 from the neighbouring groups, but in AmpMdesma we have a Tellen-Mactra 

 and in Lutraria a Mya-Mactra ; and yet there is no sufficient difference 

 in the animals to render their association unnatural. The genera are — 



Ccmingia. 



Amphidesma. 



Ungulina. 



Mesodesma. 



Crassatella. 



Gnathodon. 



Mactra. 

 Lutraria. 



triassic sandstone at Exmouth. Wherever we have a seacoast of mountain limestone, the surface 

 of the rocks is almost invariably riddled by Saxicava. The whole front of the Plymouth break- 

 water has been attacked by it, and much alarm for its safety excited." — Forbes and Hartley, 

 Brit. Moll., vol. i. p. 138. 



