CONCHIFERA. 425 
large and oval. Jossil, 15 species. Silurian, United States, 
Europe. 
P 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 
squamosus, 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. 
DREISSENA, Van Beneden. 
Litymology, dedicated to Dreyssen, a Belgian physician. 
Synonyms, Mytilomya, Cantr. Congeria, Partsch. Ticho- 
gonia, Rossm. 
Type, D. polymorpha, Pl. XVII., Fig. 9. (Mytilus Volge, 
Chemn.) p 
Shell like Mytilus, withoutits pearly ining; 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 ‘Fig. 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, 
me 
* Hall and Salter employ the name Orthonotus for such shells as Solen constrictus 
Sandb. Devonian, Germany; Sanguinolites anguliferus, M’Coy, U. Silurian, Kendal : 
and Solenopsis minor, M’Coy, Carb. limestone, Ireland. M. D’Orbigny has nistaked 
the plaits for teeth, and placed the genus with Nucula. The recent M. plicata, Lam., 
fvora Nicobar Islands, has the same long, straight back and plaited dorsal region. 
