432 MANUAL OF THE MOLLUSCA, 
tooth on each valve. Right valve plate with an obscure tooth 
behind the central one. Anterior muscular scar deep; pallial 
impression entire. 
Fossil, 6 species. Devonian, Britain. 
PSEUDAXINUS, Salter. 
Type, P. (Anodontopsis) securiformis, M‘Coy, and P. trigonus. 
Shell thin, edentulous, convex with prominent umbones, and 
a strong posterior carinated edge; beaks anterior; no lunette. 
LYRODESMA, Conrad, 1841. 
Type, Li. plana, New York. 
Synonym, Actinodonta, Phil. 
Shell trigonia-shaped, rather elongated, with a striated pos- 
terior area; hinge with several (5—9) radiating teeth, striated 
ACIOSS 5 tieamont external. 
Fossil, 4 species. Lower Silurian: Canada, United States, 
Britain. 
FAMILY VI.—UNIONIDA. Naides. 
Shell usually regular, equivalve, closed; structure nacreous, 
with avery thin prismatic-cellular tayer beneath the epidermis ; 
epidermis thick and dark; ligament external, large and pro- 
minent ; marginseven; anterior hinge-teeth thick and striated, 
posterior laminar, sometimes wanting; adductor scars deeply 
impressed ; pedal scars three, distinct, two behind the anterior 
adductor, one in front of the posterior. 
Animal with the mantle-margins united between the siphonal 
orifices, and, rarely, in front of the branchial opening; anal 
orifice plain, branchial fringed; foot very large, tongue-shaped, 
compressed, byssiferous in the fry; gills elongated, sub-equal, 
united posteriorly to each other and to the mantle, but not to 
the body; palpi moderate, laterally attached, striated imside ; 
lips plain. Sexes distinct. 
The river musselsare found in the pondsand streams of all parts 
of the world. In Europe the speciesare few, though specimens 
are abundant; in North America both species and individuals 
abound. All the remarkable generic forms are peculiar to 
South America and Africa. Two of these are fixed, and irre- 
gular when adult, and have been placed with the chamas and 
oysters by the admirers of artificial systems ; fortunately, how- 
ever, M. D’Orbigny has ascertained that the IMulleria, which 
