470 MAITOAL OP THE MOLIITSCA, 



Type^ P. globosus (Megadesmus), J. Sowerby, in Mitchell's 

 Australia. 



8hell oval, ventricose, very tliick ; ligament large, external ; 

 lunette more or less distinct; binge-line sunk; teetb 1 or 2 (?) 

 in each valve ; adductor impressions deep ; anterior pedal scar 

 distinct ; pallial line broad and simple, or with a very shallow 

 sinus. 



Fossil, 5 species. Devonian ? New South Wales, Tasmania. 



PachYrisma, Morris and Lycett. 



Etymology, pachus, thick, ereisma, support. 



Type, P. grande, Morris and Lycett. 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 ?) ; ligamental plates 

 short and deep. 



Opis, Defrance. 



Example, 0. lunulata, PI. XIX., Pig. 24. {Opis, a name of 

 Artemis.) 



Shell strong, ventricose, cordiform, obliquely keeled ; beaks 

 prominent, incurved, or sub -spiral ; cardinal teeth 1.1; lu,nule 

 distinct. 



Fossil, 42 species. Trias — Chalk. Europe. 



Caedinia, Agassiz. 



Etymology, cardo-inis, a hinge. 



Type, C. Listeri, PL XIX., Pig. 23. 



Synonyms, Thalassides, Berger, 1833 (no description). Sine- 

 muria, Christol. Pachyodon, Stutch. (not Meyer nor Schum). 

 Pronoe, Agassiz. 



Shell oval or oblong, attenuated posteriorly, compressed, 

 strong, not pearly, marked by lines of growth ; ligament 

 external ; cardinal teeth obscure, laterals 1 — 0, — 1, remote, 

 prominent ; adductor impressions deep ; pallial line simple. 



Fossil, 71 species. Silurian — Inferior Oolite. Europe ; along 

 with marine shells. 



Sub-gerMs ? AntJiracosia, King, 1844 ; Unio sub-constrictus, 

 Sowerby. (Carbonicola, M'Coy, 1856.) Upper Silurian — Garb. 

 40 species. They occur in the valuable layers of clay-ironstone 

 called "mussel-bands," associated with Nautili, Biscince, &o. 



