230 



MANUAL OF THE MOLLUSCA. 



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

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

 tooth-like, prominent and curved. 



Fossil, 100 sp. L. Silurian Garb. Arctic America, U.S. S. America 

 Falkland Ids. Europe, Thibet. 



? Sub-genera, Orthisina, D'Orb. 0. anomala, Schl. Fig. 148 

 Pronites (ascendens) and Hemipronites, Pander. Shell impunctate ? wii 

 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. pelargonatus (Streptorhynchus, King) 

 from the Magnesian limestone, 0. senilis, Garb 

 limestone, and some Devonian species, have the 

 beak twisted, as it if had been attached ; there 

 is no foramen. 



STROPHOMENA, Blainville.* 

 Etym. Strophos bent, mene crescent. 



Ex. S. rhomboidalis, PI. XV. fig. 24. (= Leptaena depressa, Sby.) 

 Syn. Leptsena (depressa) Dalnian. Leptagonia, M'Coy. Enteletes, ] 

 Shell semi-circular, widest at the hinge-line, concavo-convex, depn 

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

 gressively covered by a convex pseudo-deltidium ; umbo depressed, rarely $] 

 perforated, in young shells, by a minute foramen (fig. 149, e) ; musculai 

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

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

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

 the add actor muscle. 



Fig. 148, Orthisina. 



Fig. 149. Ventral valve. Dorsal valve. 



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



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



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

 unknown or imaginary fossil ; it has, however, been adopted both in America an< 

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



