coKcsirEEA. 423 



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 the large size of tbe anterior muscular 

 impression; and tbe subcentral umbones distinguish it from 

 Modiolarca. 



Hoplompfilus {crSLSsus), Sdbgr. Devonian, Nassau. Shell with 

 a muscular plate in the umbo, like Septifer. The Mytilus 

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



HiPPOMYA. Salter. 



Shell gibbous, with anterior inflated close beaks, a long 

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

 strong sinus from the inflated posterior ridge and slope. 



Fossil, 1 species. Devonian. 



Dreisseista, Van Beneden. 



Etymology, dedicated to Dreyssen, a Belgian physician. 



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

 gonia, Eossm. 



Type, D. polymorpha, PL XYII., Fig. 9. (Mytilus Yolgse, 

 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 with the mantle closed ; byssal 

 orifice small; and siphon very small, coni- 

 cal, plain, branchial prominent, fringed ^ig* 218. Dreissena. 

 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 Mr. J. Sowerby in the Surrey docks, 



* Hall and Salter employ the name Ortlionotus for sneli shells as Solen constrictus 

 Sandb. Devonian, G-ermanr : Smiguinolites angidiferus, M'Coy, tJ. Silurian, Kendal ; 

 and Solenopsis minor. M'Coy, Garb, limestone, Ireland. M. D'Orbigny has mistaken 

 the plaits for teeth, and placed the genus with Nucula. The recent M. plicata^ Lam., 

 from Nicohar Islands, has the same long, straight back and plaited dorsal region. 



