230 



MANUAL OF THE MOLLUSCA. 



Shell transversely oblong, radiately striated or plaited, bi-convex, hinge- 

 line narrower than the shell, cardinal process simple, brachial processes 

 tooth-like, prominent and curved. 



Tossil, 100 sp. L. Silurian — Carb. Arctic America, U.S. S. America, 

 Falkland Ids. Europe, Thibet. 



? Sub-(/enera, Orthisina, D'Orb. 0. anomala, Schl. Fig. 148. Syn. 

 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. L. Silurian, Europe. 



0. pelargonatiis (Strejitorhynchus, King) 

 from the Magnesian limestone, 0. senilis, Carb 

 limestone, and some Devonian species, have the 

 beak twisted, as it if had been attached ; there 

 is no foramen. 



Fig. 14S, Orthisina. 



Strophomena, Blainville.* 

 Etym. Slrophos bent, ?nene crescent. I 



J'.r. S. rhomboidalis, PI. XV. fig. 24. (= Leptsena depressa, Sby.) 

 Syn. Leptcena (depressa) Dalman. Leptagonia, M'Coy. Euteletes, Fischer. 

 Shell semi-circidai-, widest at the hinge-line, concavo-convex, depressed, 

 radiately striated; area double; ventral valve with an angular notch, pro- 

 gressively covered by a convex pseudo-deltidium ; umbo depressed, rarely (?) 

 perforated, in young sheUs, by a minute foramen (fig. 149, e) ; muscular 

 depressions 4, central pair narrow, formed by the adductor : external pair 

 {m) fan-like, left by the cardinal and pedicle muscles ; dorsal valve with a 

 bi-lobed cardinal process, between the dental sockets, and four depressions for 

 the adductor muscle. 



Fig. 149. Veritral valve. Dorsal valve. 



Interior of S. analoga, Carb. limestone (after King). 



e, foramen ; t, teeth ; o, ovarian spaces; b, brachial pits ? 



* The name Strophomena (rugosa) was originally given by Rafinesque to some 

 unknown or imacinary fossil; it has, however, been adopted both in America and 

 Europe for the group typified by S. alternata and. planumbona. 



