CONCHIFEEA. 469 



? Snh-gemis. I^edonia, Eouaiilt, BuU Soc. Geol., 8, 362. Shell 

 oval, tumid ; hinge with, cardinal and posterior teeth ; anterior 

 adductor bounded by a ridge. Fossil, Lower Silurian, Brittany, 

 Portugal. (Sharpe.) 



Fossil, 5 species. Lower Silurian — Trias. United States, 

 Europe, New South "Wales, Tasmania. 



? Caedilia, Deshayes. 



Tijjoe, C. semisulcata, PI. XVIII. , Fig. 18. 



Synonym, Hemicyclonosta, Deshayes. 



Shell oblong, ventricose, cordate; beaks prominent, sub- 

 spiral ; hinge with a small tooth and dental pit in each valve ; 

 ligament partly internal contained in a spoon-shaped inflection ; 

 anterior muscular scar long, with a pedal scar above ; posterior 

 adductor impression on a prominent sub-spiral plate; pallial 

 line simple. 



Distribution, 2 species. Chinese Sea, Moluccas. 



Fossil, 2 species. Eocene — . Prance, Piedmont. 



MEG.viODOX, J. Sowerby. 



Ti/2^e, M. cucullatus, PI. XIX., Pig. 19. [Megas, large, odous, 

 tooth.) 



Shell oblong, smooth or keeled ; ligament external ; hinge- 

 teeth 1.2, thick; laterals 1.1, posterior; anterior adductor 

 impression deep, with a raised margin, and a small pedal scar 

 behind it. 



In the typical species the beaks are sub.- spiral, the lateral 

 teeth obscure, and the posterior adductors bounded by prominent 

 ridges. 



Fossil, 14 species. Upper Silurian — Devonian. United States, 

 Europe. 



Suh-f/enera. ? Ooldfussia (nautiloides), Castlenau. Umbones 

 spiral ; anterior side concentrically furrowed ; posterior side 

 with two oblique ridges. Fossil, Silurian, United States. 



Megaloma (Canadensis), Hall, 1852. Upper Silurian, Canada. 

 Umbones very thick, hinge-teeth rugged, almost obliterated 

 with age; posterior lateral teeth 1.1 ; no muscular ridges. 



Pachydomus (Morris), J. Sowerby. 



Etymology ^ pachus, thick, demos, house. 



Synonyms, Astartila, Dana. ? Cleobis (grandis^, Dana. 

 P Pyramus (ellipticus), D. =Notomya, M'Coy. 



