ANATINID-a;. 155 



CuNEAMYA, Hall and Whitfield, 1815. 



Distr. — G. Mianiiensii^, H. and W. (cxx, 6). Silurian; Ohio. 



Thin, fragile, bivalve shells, with ventricose valves, and strong, 

 larominent, incurved beaks ; cardinal line straight or gently 

 curved ; hinge edentulous ; valves united by an external liga- 

 ment of greater or less extent, posterior to which the margins 

 of the valves overlap each other to the extent of the cardinal 

 line ; margins of the valves inflected along the cardinal border, 

 forming a narrow escutcheon posterior to the beaks, and ante- 

 riorly a well-defined lunule is situated below the beaks ; adductor 

 muscles, at least two, are anterior and posterior; pallial line 

 simple. 



Orthodesma, Hall and AVhitfield, 1815. 



Distr. — 0. recta, H. and W. (cxx, 1). Hudson Kiv. Gr. ; Ohio. 



More or less elongate, bivalve shells, having the hinge-line 

 straight and generally extended posterior to the beaks, but con- 

 tracted or bent beneath or anterior to them ; hinge-plate appar- 

 ently edentulous ; valves united hy an external ligament extending 

 to a greater or less distaLce along the posterior cardinal margin. 

 Posterior muscular scar elongate-ovate, anterior scar smaller, 

 both faintly marked ; pallial line simple. Shells thin, marked 

 externally with irregular concentric plicae. 



Ilionia, Billings, 1874. 



Distr. — 3 sp. Sil., Devon. ; IT. S. I. sinuata, Hall. 



Shell (cast) irregularly ovate, compressed or sublenticular, one 

 extremity larger than the other ; beaks turned towards the larger 

 end, which is therefore supposed to be anterior ; a concave 

 depression commences on the umbones and extends downwards 

 to the posterior ventral margin ; a large subovate muscular 

 impression in the upper half of the posterior extremity ; close 

 under the beaks in front there appears to have been vS) short 

 escutcheon ; no teeth. 



Clarkia, Koninck, 1878. 



Distr. — G. Myiformis., Dana. Garb. ; N. S. Wales. 



Shell elongated, equivalve, gaping at its anal extremity, rather 

 thick, with growth-lines ; ligament external; hinge thick, callous, 

 with a small tooth in each valve immediately under the beak; 

 interior surface smooth ; adductor and pedal impressions sep- 

 arate, the first large, oval, but slightly marked, the last stronger 

 and small ; pallial line nearly simple, slight. 



[RiBEiRiA, Sharpe, 1853. 

 Distr. — Fossil, 4 sp. L. Silurian ; Portugal, Canada, England. 

 Shell gaping at both ends ; subovate, rounded in front, elon- 



