274 AVICULID^. 



Shell subeqni valve, 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. 



PosiDONOMYA, Bronn, 1837. 



Etym. — Foneidon, Neptune. 



Syn. — Posidonia, Br., 1838 (not Konig). Aulocomya, Stein- 

 mann, 1881. 



Diatr. — Fossil, 50 sp. Lower Silurian — Trias ; United States, 

 Europe. P. Becheri, Bronn (cxxx, 46). 



Shell thin, equivalve, compressed, earless, concentrically 

 furrowed ; hinge-line short and straight, edentulous. 



Rhynchopterus, Meek, 18f)4. 



D^■^•^r.—i?. o&esws. Meek (cxxx, 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 stride only. 



This genus greatl}^ resembles one of the mesozoic Aviculse 

 with smooth surface, differing from them b}^ the absence of the 

 posterior wing. The shell would also appear to have a close 

 resemblance to Myalina. ' 



MoNOPTERiA, Meek and Worthen, 1866. 



Distr. — Carboniferous; United States. M. 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 beloAv the beak, 

 without a byssal emargination, but apparently slightl}^ gaping 

 within the lunule ; muscular scars ver}^ faint; 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 Amphicoelia, which, they say, " was proposed as a sub- 

 genus under Leptodomus, to which it is not nearly allied. It 

 evidently belongs to the Aviculidae, near Pterinea, though 

 apparently generically distinct " (see p. 275). 



Subfamily AMBONYCHIINJE. 



Shell equivalve, beaks sharp, at the anterior end of the long 

 straight hinge ; ligament linear, parallel with the hinge-plate ; a 



