470 MANUAL OF THE MOLLUSCA, 
Type, P. globosus (Megadesmus), J. Sowerby, 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 yalve ; adductor impressions deep; anterior pedal scar 
distinct; pallial line broad and simple, or with a yery shallow 
sinus. 
Fossil, 5 species. Devonian? New South Wales, Tasmania. 
PAcHYRISMA, Morris and Lycett. 
Etymology, pachus, thick, ereisma, support. 
Type, P. grande, Morrisand 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 ?); hgamental plates: 
short and deep. 
Opis, Defrance. 
Example, O. lunulata, Pl. XTX., Fig. 24. (Opis,a name of 
Artemis. ) 
Shell strong, ventricose, cordiform, obliquely keeled; beaks 
prominent, incuryed, or sub-spiral; cardinal teeth 1.1; lunule 
distinct. 
Fossil, 42 species. Trias—Chalk. Europe. 
CARDINIA, Agassiz. 
Etymology, cardo-inis, a hinge. 
Type, C. Listeri, Pl. XTX., Fig. 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, 0—1, remote, 
prominent; adductor impressions deep; pallial line simple. 
Fossil, 71 species. Silurian—Inferior Oolite. Hurope; along 
with marine shells. 
Sub-genus? Anthracosia, King, 1844 ; Unio sub-constrictus, 
Sowerby. (Carbonicola, M‘Coy, 1856.) Upper Silurian—Carb. 
40 species. They occur in the valuable layers of clay-ironstone 
called ‘‘mussel-bands,”’ associated with Nautili, Discine, &e. 
