COXCTIlFEEAs 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, has the mantle open; 

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

 impression; and the subcentral umbones distinguish it from 

 Modiolarca. 



Hoplomytilus (crassus), Sdbgr. Devonian, Nassau. Shell with 

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

 equamosus, 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. 



Dreissexa, Yan Beneden, 



Etymology, dedicated to Dreyssen, a Belgian physician. 



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

 gonia, Eossm. 



Type, D. polymorpha, PI. XYIL, 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 ^'S- 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 Siu-rey docks, 



* Hall and Salter emplo\ the name Orthonotus for such shells as SoJen comttrictus ^ 

 Sandb. Devonian, Germany : Satu/uinohtes nnguliferus, M'Coy, U. Silurian, Kendal ; 

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

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

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



