302 MANUAL OF THE MOLLUSCA. 



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 sp. U. Silurian Devonian ; U. States, Europe. 



Sub-genera. ? Goldfussia (nautiloides) Castlenau. Umbones spiral; 

 anterior side concentrically furrowed ; posterior side with two oblique ridges. 

 Fossil, Silurian, U. States. 



Megaloma (Canadensis) Hall, 1852. U. Silurian, Canada. Umbones 

 very thick, hinge-teeth rugged, almost obliterated with age ; posterior lateral 

 teeth 1.1 ; no muscular ridges. 



PACHYDOMUS (Morris) J. Sowerby. 



Etym. Pachus, thick, domos, house. Syn. Astartila, Dana. 



? Cleobis (grandis) Dana. ? Pyramus (ellipticus) D. = Notomya, M'Coy. 



Type, P. globosus (Megadesmus) J. Sby. in Mitchell's Australia. 



Shell oval, ventricose, very thick; ligament large, external; lunette 

 more or less distinct; hinge-line sunk; teeth 1 or 2 (?) in each valve ; 

 adductor impressions deep ; anterior pedal scar distinct ; pallia! line broad 

 and simple, or with a very shallow sinus. 



Fossil, 5 sp. Devonian ? N. S. Wales, Tasmania. 



PACHYRISMA, Morris and Lycett. 



Etym. Pachus, thick, ereisma, support. 



Type, P. grande, M. and L. Great Oolite (Bathonian) Minchinhampton. 



Shell cordate, with large sub-spiral beaks ; valves very thick near the 

 umbones, obliquely keeled ; hinge with one thick conical tooth (behind the 

 dental pit, in the right valve), a small lateral tooth close to the deep and oval 

 anterior adductor, and a posterior lateral-tooth (or muscular lamina ?) ; liga- 

 mental plates short and deep. 



OPIS, Defrance. 



Ex. 0. lunulata, PI. XIX. fig. 24. (Opis, a name of Artemis.) 

 Shell strong, ventricose, cordifonn, obliquely keeled ; beaks prominent 

 incurved or sub-spiral ; cardinal teeth 1.1 ; lunule distinct. 

 Fossil, 42 sp. Trias Chalk. Europe. 



CAEDINIA, Agassiz. 



Etym. Cardo-inis, a hinge. Type, C. Listeri, PI. XIX. fig. 23. 



Syn. Thalassides, Berger 1833 (no descr.) Sinemuria, Christol. Pachy 

 odon, Stutch. (not Meyer nor Schum.) Pronoe, Ag. 



Shell oval or oblong, attenuated posteriorly, compressed, strong, no 

 pearly, marked by lines of growth ; ligament external ; cardinal teeth ob 



