380 MANUAL OF THE MOLLUSCA. 



Orthis, Dalman. 



Etymology, orthos, straight. 

 Type, 0. rustica, PL XY., Fig. 23. 



Synonyms, Dicoelosia (biloba), King. Platystropliia (biforata), 

 King. Gonambonites (inflexa), Pander. Ortliambonites (calli- 

 gramma), Pander. 



Shell transversely oblong, radiately striated or plaited, bi- 

 convex, binge-line narrower than the shell, cardinal process 

 simple, brachial processes tooth-like, prominent and carved. 



i^ossi7, 154 species. Lower Silurian — Carb. Arctic America, 

 United States, South America, Falkland Islands, Europe, 

 Thibet. 



? Sub-genera. Orthisina, D'Orbigny. O. anomala, Schl. 

 Fig. 183. Synonyms, Pronites . (ascendens) and Hemipronites, 

 Pander. Shell impunctate ? widest at 

 the hinge-line ; cardinal notch closed, 

 byssal notch {fissure) covered by a 

 convex pseudo-deltidium, sometimes 

 perforated by a small round foramen. 

 Fossil, Lower Silurian, Europe. 



0. pelargonatus (Streptorhynchus, 



King), from the Magnesian limestone, 



Fig. 183. Ort/usma. q senilis, Carb. limestone, and some 



Devonian species, have the beak twisted, as if it had been 



attached ; there is no foramen. 



Strophomena, Blainville.* 



Etymology , strophos, bent, mene, crescent 



Examples, S. rhomboidalis, PL XV., Fig. 24. (=Productus 

 depressus, Sby.) 



Synonyms, Leptsena (rugosa), Dalman. Leptagonia, M'Coy. 

 Enteletes, Fischer. 



Shell semicircular, widest at the hinge-line, concavo-convex, 

 depressed, radiately striated ; area double ; ventral valve with 

 an angular notch, progressively covered by a convex pseudo- 

 deltidium ; umbo depressed, rarely (?) perforated, in young 

 shells, by a minute foramen (Fig. 184, e) ; muscular depressions 

 4, central pair narrow, formed by the adductor ; external pair 

 [m) fan-like, left by the cardinal and pedicle muscles ; dorsal 



* The name Sfrophomcnn (rugosa) was originally given by Rafincsqiie to some un- 

 known or imaginary fossil; it has, however, been adopted both in America and 

 Europe for the gi'oup typified by S. alternata and pUinumbona. 



