378 MOLLUSCA SUB-KINGDOM VI 
Anodonta, Cuv. Valves thin, and armature of the hinge obsolete ; lives in still, 
muddy water. Tertiary and Recent. 
Family 29. Mutelidae. Gray. 
Shell and anatomy resembling the Unionidae, without pseudocardinals and laterals ; 
having, when dentiferous, an arregularly Taxodont hinge armature, generally partly 
closed mantle lobes, a complete branchial septum, more complete siphons, and with a 
nepronic stage represented by a Lasidiwm, resulting in unsculptured beaks for the adult 
shell. Cretaceous to Recent. 
Spatha, Lea. Elongated, inequilateral, with a short edentulous hinge. Upper 
Cretaceous of Provence, and Recent in South Africa. 
Leila, Gray ; Aplodon and Plagiodon, Spix ; Mycetopus, VOrb.; Mutela, Scopoli ; 
and Iridina, Lam. Recent. 
Family 30. Etheriidae. Lamarck. 
Shell sessile, irregularly modified by adherence to other bodies, nacreous, with a 
tendency to cellularity of structure; edentulous; ligament amphidetic, parivineular, 
deeply sunken, with a large internal resilium, modified by the distortion of the valves ; 
young regular, equivalve, dimyarian ; the adult irregular, inequivalve, and either (1) 
monomyarian, or (2) with a very degenerate anterior adductor, or (3) with sub-equal 
adductors. Mantle lobes wnited only for the anal siphon ; foot degenerate or absent in 
the adult ; young byssiferous ; station fluviatile. Pleistocene and Recent. 
The young shell of Bart/lettia has well-marked nymphae and internal resilium. The 
relationship of the Naiades to Pteria renders the remarkable resemblance of the adult Mulleria 
to Ostrea less surprising, since Ostrea is now known also to be derived from the Pteriidae. 
Etheria, Lam. Ostreiform, attached to rocks in African rivers. Also Pleistocene 
of West Africa. 
Mulleria, Ferussac; Bartlettia, Adams. Recent ; South American rivers. 
Superfamily 6. TRIGONIACEA. Bronn. 
Shell equivalve, inequilateral, closed, dimyarian, not alate; shell substance nacreous 
and prismatic ; hinge teeth few, sub-wmbonal, typically Schizodont ; area obscure or none ; 
ligament parivincular, opisthodetic, external ; gills filibranchiate ; mantle lobes usually 
free, but modified on the posterior edges to form functional siphons without conjunctive 
partitions ; pallial line usually simple; non-byssiferous, though possessing an obsolete 
byssal apparatus ; young without a distinct nepioniec stage ; dioecious ; marine. 
Family 31. Lyrodesmidae. Ulrich. 
Shell with the hinge armature radiating fan-like from below the umbones ; teeth five 
to nine; pallial line feebly sinuate or simple. Silurian. 
Lyrodesma, Conr. (? Actinodonta, Phil.) Shell oval, cardinal border narrow, without 
ligamentary area. Silurian; America and Europe. 
Family 32. Trigoniidae. Lamarck. 
Shell with few hinge teeth (2), the mantle lobes wholly free, but so applied to each other 
in life as to form functional siphons ; pallial line simple. Devonian to Recent. 
