380 MANTTAL OF THE MOLLTJSCA, 



Orthis, Dalman. 



Etymology, ortJios, straight. 

 Type, 0. rustica, PI. XV., Fig. 23. 



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

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

 gramma), Pander. 



Shell transversely oblong, radiately striated or plaited, bi- 

 convex, hinge-line narrower than the shell, cardinal j)roces3 

 simple, brachial processes tooth-like, j)roininent and carved. 



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

 United States, South America, Falkland Islands, Europe, 

 Thibet. 



? Suh-genera. Orthisina, D'Orbigny. 0. 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 Siliuian, Europe. 



0. pelargonatus (Streptorhynchus, 



King), from the Magnesian limestone, 



Fig. 183. Orthisina. q^ semlis, Carb. limestone, and some 



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



attached ; there is no foramen. 



Steophomeka, Blainville . * 



Etymology, strophos, bent, mcne, crescent ■ 



Examples, S. rhomboidalis, PI. XY,, 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 j^air 

 (?n) fan-like, left by the cardinal and pedicle muscles ; dorsal 



* The name Strophomena (rugosa) was originally given by Rafinesqne to some un- 

 known or imnginary fossil; it has, however, been adopted both in Ainer'ca an** 

 Eui'ope for 11) e group typified by S. alternata an^ planumdonn. 



