378 MOLLUSCA 



SUB-KINGDOM VI 



Anodonta, Cuv. Valves thin, and armature of the hinge obsolete ; lives in still, 

 muddy water. Tertiary and Kecent. 



Family 29. Mutelidae. Gray. 



Shell and anatomy resembling the Unionidae, without pseudocardinals and laterals ; 

 having, when dentiferous, an irregularly Taxodont hinge armature, generally partly 

 closed mantle lobes, a complete branchial septum, more complete siphons, and with a 

 nepionic stage represented by a Lasidium, resulting in unsculptured beaks for the adult 

 shell. Cretaceous to Eecent. 



Spatha, Lea. Elongated, inequilateral, with a short edentulous hinge. Upper 

 Cretaceous of Provence, and Eecent in South Africa. 



Leila, Gray ; Aplodon and Plagiodon, Spix ; Mycetopus, d'Orb. ; Mutela, Scopoli ; 

 and Iridina, Lain. 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, parivincular, 

 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 united only for the anal siphon ; foot degenerate or absent in 

 the adult ; young byssiferous ; station fluviatile. Pleistocene and Recent. 



The young shell of Sartlettia has well -marked nyraphae 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-umbonal, 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 nepionic 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 (), the mantle lobes wholly free, but so applied to each other 

 in life as to form functional siphons ; pallial line simple. Devonian to Recent. 



