432 MANUAL OF THE MOLLUSCA. 



tooth on each valve. Eight 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. 



Lyeodesma, Conrad, 1841. 



Type, L. 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 

 across ; ligament external. 



Fossil, 4 species. Lower Silurian : Canada, United States, 

 Britain. 



Family YI. — Unionidje. Naides. 



Shell usually regular, equivalve, closed ; structure nacreous, 

 with a very thin prismatic-cellular' ^ayer beneath the epidermis; 

 epidermis thick and dark ; ligament external, large and pro- 

 minent ; margins even ; 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 inside; 

 lips plain. Sexes distinct. 



The river mussels are found in the ponds and streams of all parta 

 of the world. In Europe the species are 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 Ilulleria, which 



