274 AVICULIDA. 
Shell subequivalve, obliquely oval, depressed, posteriorly 
slightly eared, anterior side short; surface radiated; beaks 
depressed, submedian; cardinal line linear, callous, without 
teeth, with an inflexion for the passage of the byssus. 
PosiponomyA, Bronn, 1837. 
Etym.— Poseidon, Neptune. 
Syn.—Posidonia, Br., 1838 (not Kénig). Aulocomya, Stein- 
mann, 1881. 
Distr.—F ossil, 50 sp. Lower Silurian—Trias ; United States, 
Kurope. P. Becheri, Bronn (cexxx, 46). 
Shell thin, equivalve, compressed, earless, concentrically 
furrowed ; hinge-line short and straight, edentulous. 
RuyNcuHoprervs, Meek, 1864. 
Dixtr.— R. obesus, Meek (exxx, 47). Triassic; Nevada. 
Shell obliquely elongated, tumid, anteriorly narrow, with a 
small acute ear, posterior side uniformly and slightly curved, 
not distinctly winged; hinge-line straight, toothless, slightly 
thickened ; surface covered with concentric striew only. 
This genus greatly resembles one of the mesozoic Aviculz 
with smooth surface, differing from them by the absence of the 
posterior wing. The shell would also appear to have a close 
resemblance to Myalina. 
Monopreria, Meek and Worthen, 1866. 
Distr.— Carboniferous; United States. MM. gibbosa, M. and W. 
(Cxxx,/48); 
Obliquely subquadrate, subequivalve, moderately convex, with 
a large posterior wing, being below the end generally insinuated, 
anterior wing obsolete or nearly so, impressed below the beak, 
without a byssal emargination, but apparently slightly gaping 
within the lunule ; muscular sears very fsint; hinge edentulous; 
ligamental area with a few longitudinal furrows. 
This has been proposed as a subgenus of Pterinea, but it 
appears to have no internal hinge-ribs which characterize that 
genus. The authors speak of certain ligamental furrows extend- 
ing internally, but they are not shown in the figure of the cast, 
which in that part appears perfect. 
Meek and Worthen also refer to the relation of Monopteria to 
Hall’s Amphiccelia, which, they say, ‘was proposed as a sub- 
genus under Leptodomus, to which it is not nearly allied. It 
evidently belongs to the Aviculide, near Pterinea, though 
apparently generically distinct ” (see p. 275). 
SuBFAMILY AMBONYCHIIN. 
Shell equivalve, beaks sharp, at the anterior end of the long 
straight hinge; ligament linear, parallel with the hinge-plate; a 
