303 MANUAL OF THE MOLLUSCA. 



laterals 1.1, posterior; anterior adductor impression deep, with a raised , 



margin ; and a small pedal scar behind it. I 



In the tj^ical species the beaks are sub-spiral, the lateral teeth obscure, i 



and the posterior adductors bounded by prominent ridges. j 



Fossil, 14 sp. U. Silurian — Devonian; U. States, Europe. 



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

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



Fossil, Silm-ian, U. States. | 



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



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



teeth 1.] ; no muscular ridges. j 



Pachtdomus (Morris) J. Sowerby. t 



Etyni. Pachus, thick, domos, house. Sijn. Astartila, Dana. 



? Cleobis (grandis) Dana. ? Pyramus (eUipticus) D. = Notomya, M'Coy. 1 



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 ; pallial line broad x 



and simple, or with a very shallow sinus. \ 



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



Pachyhisma, Morris and Lycett. '^ 



Ftym. 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. O. lunulata, PI. XIX. fig. 24. {Opis, a name of Artemis.) 

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

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

 Fossil, 42 sp. Trias — Chalk. Europe. 



Cakdinia, Agassiz. j 



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



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, not J 



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



