MAEYLAND GEOLOGICAL SURVEY 627 



Family DREISSENIIDAE 



Genus DREISSENA Van Beneden 

 [Ann. Sci. Nat, ser. 2, vol. iii, 1835, p. 193, pi. viii] 



Type. Mylilus polymorplius Pallas. 



Equivalve, inequilateral, slightly gaping as a rule, mytiliform in out- 

 line ; umbones acute, terminal, bent a little forward ; anterior area differ- 

 entiated by a more or less obtusely angulated keel which runs from the 

 umbones to the anterior ventral margin ; external surface smooth or incre- 

 mentally sculptured ; ligament internal or submarginal, lodged in a shal- 

 low groove, which extends more than a third of the way down to the base: 

 angle between the umbones bridged by a transverse septum upon which 

 the anterior and pedal adductors are mounted and from which, in the right 

 valve, a small dentiform process sometimes projects ; posterior adductor 

 scar moderately large, well down towards the base; pallial line rather 

 obscure, entire. 



Dreissena is very like Mytilus in general aspect, so much so, indeed, that 

 there is probably a considerable amount of confusion between the two 

 genera in the earlier described species. Many authorities maintain that 

 the genus is not initiated until the Early Tertiary. Henry Woods, 1 how- 

 ever, has reported a species, I>reissensia lanceolata (Sowerby) Woods from 

 the Cretaceous of England, and has so adequately figured it that there 

 can be no doubt about the correctness of his determination. 



The shell differs most conspicuously from that of Mytilus in the devel- 

 opment of the septum in the umbonal angle and the more internal liga- 

 ment. The animal differs from that of Mytilus in the closed mantle and 

 the two distinct siphons. All of the recent species are denizens of fresh or 

 brackish water. 



Etymology: Named in honor of Dreissens, a Belgian physician. 

 1 1900, Mon. Cret. Lamellibranchia, England, Paleontographical Soc., Lon- 

 don, pt. ii, p. 110, pi. xviii, figs. 13-15; pi. xix, figs. 1-11. 



