2*12 AVICULIDiE. 



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," — Carpenter. A. Mosquiensis, 

 Keyserl. (cxxx, 35). 



Pteroperna, Morris and Lycett, 1850. 



Distr. — Fossil, 3 sp. Bath Oolite ; Britain, France. P. cos- 

 tulata, M. and L. (cxxx, 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. 



Distr. — Fossil, 6 sp. Upper Trias — L. Lias ; Austria, Bavaria, 

 Himalayas. C. gryphseata, Miinst. (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. 



Pseudomonotis, Beyrich, 1862, 



Syn. — Eumicrotis, Meek, 1864, 



Bistr. — Devon., Triassic, Jvirassic. P. speluncaria, Miinst. 

 (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 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. 



Pterinea, Goldfuss, 1832. 



Distr. — Fossil, 32 sp. Lower Silurian — Carb. ; United States, 

 Europe, Australia. F. Isevii^^ Goldfuss (cxxx, 43). 



Shell thick, rather inequi valve, very oblique and broad W 

 winged ; beaks anterior, sinus shallow ; hinge-area long, straight, 

 narrow, striated lengthwise; anterior teeth few, radiating; pos- 



