328 STROPHOMENIDA. 
trunks in the dorsal valve, two in the ventral, the external 
branches turned outwards and backwards, inclosing wide ova- 
rian spaces. Indications have been observed, in several genera, 
of horizontally coiled spiral arms; the space between the 
valves is often very small. The shell-structure is punctate, 
except in a few instances, where the original texture is probably 
obliterated. 
Orruis, Dalman, 1827. 
ELtym.— Orthos, straight. 
Syn. —Orthambonites, Pander, 1830. Schizophoria, King, 1850. 
Distr.—Fossil, 300 sp. Lower Silurian—Carb.; Arctic Am., 
United States, South America, Falkland Islands, Europe, Thibet. 
O. striatula, Schloth. (exxxviii, 37-39). 
Shell transversely oblong, radiately striated or plaited, bicon- 
vex, hinge-line narrower than the shell, cardinal process simple, 
br achial | processes tooth-like, prominent and curved. 
BILOBITES, Linn., 1775. (Diccelosia, King, 1850.) Ventral 
margin deeply cut out in the middle, forming two lobes in each 
valve; two strong curved crural processes from the hinge-margin 
of the smaller valve. Silurian; Gotland and U.S. Os biloba, 
Linn. 
PLATYSTROPHIA, King, 1850. Shell somewhat transverse, 
nearly equivalve, with a long, straight hinge-line; both valves 
very convex, radially ribbed, with an area and trigonal deltidial 
opening ; beaks curved, approaching ; large valve with a deep 
median sinus. Silurian, Carboniferous. O. lynx, Hichw. 
(ecxxxviil, 40). 
ENTELETES, Fischer, 1830. (Choristites, Fischer [part], 1825.) 
Like Platystrophia, but the hinge-mar gin short ; surface coar sely 
wrinkled, and finely radially sculptured. OF Lamarcki, Fischer. 
Carboniferous ;- Russia. 
MYSTROPHORA, Kayser, 1871. Shell like Orthis, but the small 
valve with a very high median septum, which reaches to the 
other valve, and so divides the closed shell into two chambers ; 
the two tooth-plates are united to the septum. Silurian, Devo- 
nian. O. Lewisti, Davids. Davidson makes this a synonym of 
Skenidium, Hall. 
STREPTORHYNcHUS, King, 1850. 
Etym.—Strepto, I bend or twist; rhynchos, a beak. 
Syn.—Hipparionix, Vanuxem, 1842. Orthotetes (Evans), 
Fisch. (part), 1829. 
Disir.—6 sp. Sil_—Perm.; Europe, Asia, America, and 
Australia. S. Devonica, d’Orb. 
Shell inequivalved, conv ex or concavo-convex, externally 
striated ; hinge-line rather shorter than the width of the shell : 
dorsal valve ‘semicircular, with a small narrow area. Ventral 
