272 AVICULIDA. 
Europe. ‘In A. cygnipes we find no trace of prismatic-cellular 
structure or nacre, but the coarsely corrugated and somewhat 
tubular structure of the Pectens.”—CaARpentEer. A. Mosquiensis, 
Keyserl. (cxxx, 35). 
PrEROPERNA, Morris and Lycett, 1850. 
Distr.—Fossil, 3 sp. Bath Oolite; Britain, France. P. cos- 
tulata, M. and LL. (exxx, 39): 
Shell with a long posterior wing; hinge-line bordered by a 
groove; anterior teeth numerous, minute : posterior one or two 
long, nearly parallel with the hinge-margin. 
An important external character is the presence of a longitu- 
dinal ridge on the outside of the wing. 
CASSIANELLA, Beyrich, 1861. 
Syn.—Gryphorhynchus and Acinophorus, Meek, 1864. 
Disir.—Fossil, 6 sp. Upper Trias—L. Lias; Austria, Bavaria, 
Himalayas. C. grypheata, Munst. (cxxx, 40, 41). 
Shell thick, subhemispherical; right valve flat or concave, the 
left very gibbous; no defined byssal sinus. Umbones subcentral, 
hinge-line equaling the greatest length of the shell, in both 
valves with a wide well-defined cardinal area; ears subequal, not 
produced. Hinge with several small irregular teeth near the 
middle. Surface striated. 
PseuDomMonorTIs, Beyrich, 1862. 
Syn.—Eumicrotis, Meek, 1864, 
Distr.—Devon., Triassic, Jurassic. P. speluncaria, Munst. 
(cxxx, 42) 
Suborbicular or roundly oval, the right valve being usually 
more or less convex, with small, or nearly obsolete, wings and 
prominent incurved beaks ; the left is conspicuously flattened or 
slightly concave, with barely prominent beaks and with a 
straight, thickened hinge-line, sometimes provided with a flat- 
tened tooth-like projection below, and an oblique ligamental 
groove posterior to it, corresponding to a similar groove or pit 
in the other valve ; the anterior end has below the beak a narrow 
deep byssal i incision and a small, sometimes almost obsolete, ear 
above it. Posterior adductor large, subcentral, anterior minute, 
at the base of the wing; surface usually covered with radiating 
ribs. 
Me se Goldfuss, 1832. 
Distr.—Fossil, 32 sp. Lower Silurian—Carb.; United States, 
Europe, Australia. P. levis, Goldfuss (exxx, 43). 
Shell thick, rather inequivalve, very oblique and broadly 
winged ; beaks anterior, sinus shallow ; hinge-area long, straight, 
narrow, striated lengthwise ; anterior ‘teeth few, radiating ; pos- 
