880 MANUAL OF THE MOLLUSCA. 
OrTHIS, Dalman. 
Etymology, orthos, straight. 
Type, O. rustica, Pl. XV., Fig. 23. 
Synonyms, Diccelosia (biloba), King. Platystrophia (biforata), 
King. Gonambonites (inflexa), Pander. Orthambonites (calli- 
eramma), Pander. 
Shell transversely oblong, radiately striated or plaited, bi- 
conyex, hinge-line narrower than the shell, cardinal process 
simple, brachial processes tooth-like, prominent and curyed. 
Fossil, 154 species. Lower Silurian—Carb. Arctic America, 
United States, South America, Falkland Islands, Europe, 
Thibet. 
? Sub-genera.  Orthisina, D’Orbigny. O. dnomala, Schl. 
Fig. 183. Synonyms, Pronites (ascendens) and Hemipronites, 
Pander. Shell impunctate P widest at 
== the hinge-line; cardinal notch closed, 
byssal notch (/isswre) covered by a 
convex pseudo-deltidium, sometimes 
perforated by a small round foramen. 
fossil, Lower Silurian, Europe. 
O. pelargonatus (Streptorhynchus, 
King), from the Magnesian limestone, 
_ Fig. 183. Orthisina. O. senilis, Carb. limestone, and some 
Devonian species, haye the beak twisted, as if it had been 
attached ; there is no foramen. 
STROPHOMENA, Blainyille.* 
Etymology, strophos, bent, mene, crescent, 
Examples, 8. rhomboidalis, Pl. XV., Fig. 24. (=Productus 
depressus, Sby.) 
Synonyms, Lepteena (rugosa), Dalman. Leptagonia, M‘Coy. 
Enteletes, Fischer. 
Shell semicircular, widest at the hinge-line, concayo-conyex, 
depressed, radiately striated; area double; ventral valve with 
an angular notch, progressively covered by a conyex 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 Strophomena (rugosa) was originally given by Rafinesque to some un- 
known or imaginary fossil; it has, however, been adopted both in America an? 
Europe for the group typified by S. alternata and planumbona. 
