MOLLUSCA. LAMELLIBRANCHIATA. 147 



Solen. Shell very long, sub-cylindrical, straight, 

 smooth or finely striated, the dorsal and ventral margins 

 parallel. Gaping at the anterior and posterior extremities. 

 Margin of valves smooth. Umbones anterior, terminal. 

 Hinge terminal, with one cardinal tooth in each valve. 

 Ligament long, external. Anterior adductor impression 

 elongated. Pallial sinus short, square. Trias ( ? also 

 Devonian and Carboniferous) to present day. 



Glycimeris (=Panopea). Shell equivalve, inequi- 

 lateral, oblong, thick, concentrically striated, gaping at 

 each end. Ligament external on a prominent ridge. 

 One strong cardinal tooth in each valve. Pallial sinus 

 very deep. Cretaceous to present day. 



Pholadomya. Shell thin, translucent, oblong or 

 oval, ventricose, equivalve, gaping posteriorly and some- 

 times anteriorly. Anterior side short and rounded. 

 Surface with radiating ribs crossed by concentric striae. 

 Umbones prominent. Ligament external. Hinge with- 

 out teeth or with a small transverse dentary tubercle in 

 each valve. Adductor impressions very faint. Pallial 

 sinus very deep. Lias to present day. 



Goniomya. Shell ovate-elongate, nearly equilateral, 

 gaping ; surface with fine granulations and with V-shaped 

 ribs pointing ventrally. Hinge without teeth. Ligament 

 external. Adductor impressions near the dorsal margin. 

 Lias to Cretaceous. 



Myacites (= Arcomya). Shell thin, oblong, inequi- 

 lateral, ventricose, gaping at the ends; surface granulated 

 and often with concentric furrows. Umbones small, 

 anterior. Ligament external. Hinge without teeth. 

 Adductor impressions faint. Pallial line sinuous. Trias 

 to Chalk. 



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