CONCHTFEHA. 425 



large and oval. Fossil, 15 species. Silurian, United States, 

 Europe. 



? Orthonotus (pholadis), Conrad. Lower Silurian, New York. 

 Shell elongated, margins parallel, umbones anterior, back 

 plaited.* 



Myrina, Adams. Modiola pelagica, bas tbe mantle open ; 

 tbe shell is peculiar from tbe large size of tbe anterior muscular 

 impression; and tbe subcentral umbones distinguish it from 

 Mod'i'olarca. 



Hojjlomytilus {cia^ssus), Sdbgr. Devonian, 'NsiSsaM. Shell -with 

 a muscular plate in tbe umbo, like Septifer. Tbe Mytiliis 

 squamosus, Sby. Magnesian limestone, Brit., has a similar plate. 



HiPPOMYA. Salter. 



Shell gibbous, witb anterior inflated close beaks, a long- 

 cardinal edge ; anterior edge sbort, rounded, and separated by a 

 strong sinus from tbe inflated posterior ridge and slope. 



Fossil, 1 species. Devonian. 



Dreissena, Yan Beneden. 



Etymology, dedicated to Dreyssen, a Belgian physician. 



Synonyms, Mytilomya, Cantr. Congeria, Partsch. Ticho- 

 gonia, Rossm. 



Type, D. polymorpha, PI. XYII., Fig. 9. (Mytilus Yolgce, 

 Chemn.) 



Shell like Mytilus, without its pearly lining ; inner layer com- 

 posed of large prismatic shells ; um- 

 bones terminal ; valves obtusely keeled ; 

 right valve with a slight byssal sinus ; 

 anterior adductor supported on a shelf 

 within the beak ; pedal impression single, ^ 

 posterior. 



Animal vnth. the mantle closed ; byssal 

 orifice small; and siphon very small, coni- 

 cal, plain, branchial prominent, fringed ^'^S- 218. Dreusena. 

 inside ; palpi small, triangular ; foot-muscles short and thick, 

 close in front of the posterior adductor. 



D. polymorpha is a native of the Aralo-Caspian rivers ; in 

 1824 it was observed by Jklr. J. Sowerby in the Surrey docks, 



* Hall and Salter enlplo^ the name OrthiynoUts for such shells as Solen cornet r i ctus ^ 

 Sandb. Devonian, Germany ; Sanfjidnolites angulifcmx, M'Coy, U. Siluiian, Kendal ; 

 ail 1 Solcnopsis minor, M'Coy, Carb. limestone, Ireland. M. D'Orbigny has mistaken 

 the plaits for teeth, and placed the genus with Nucida. The recent M. plicatn. Lam., 

 £;."om Kicibar Islands, has the same long, straight back and plaited dorsal region. 



