CLASS I PELECYPODA 455 



müsse] {My(c tnargaritifera Liiiii.), in which tlie posterior luiiiiiiac of the liinge are 

 obsolete. It sliows in the niajority of species aniorplious, heavy, radial, pseudocardinal 

 and lateral teetli on the hinge ; the sliell is variable in form and ornamentation, some 

 species having strongly marked sexual difterences in the shell. Most of the species 

 are peaiiy, with a conspicuous brown or greenish periostracum ; the anterior adductor 

 scars are high, and the pedal scars conspicuous. 



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

 muddy water. Tertiary and Recent. 



Margaritana Schum. The pearl mussei, formerly associated with Mya and Unio^ is 

 anatomically intermediate Ijetween the Unionidae and Mutelidae. Tertiary and Eecent. 



Family 29. Mutelidae Gray. 



Shell resemhling that of the Unionidae, without pseudocardinals and laterals ; 

 having, when dentiferous, an irregularly taxodont hinge armature; heahs unsculptured ; 

 mantle lohes generally partly closed ; siphons more complete. Nepionic stage {Jcnovm 

 only in a fev) South American forms) said to he represented by a Lasidium. Cretaceous 

 to Recent. 



Spatha Lea. Elongated, inequilateral, with a short edentulous hinge. Living in 

 Africa, and doubtfully present also in the Upper Cretaceous of Provence. 



Anodontites Bruguiere (Glaharis Gray) ; Leila Gray ; Monocondylaea and Mycetopoda 

 d'Orl).; Mutela Scopoli {Iridina Lam.); and Pleiodon Conrad. Recent. Two 

 subfamilies, Mutelinae and Hyriinae, are recognised by Ortmann on the basis of 

 structural differences in the soft parts. 



Family 30. Etheriidae Lamarck. 



Shell sessile, irregularly modified by adherence to other bodies, nacreous, with a 

 tendency to cellularity of structure ; edentulous; ligament amphidetic, parivinculary 

 deeply sunken, with a large internal resilium, modified by the distortion of the valves ; 

 young regulär, equivalve, dimyarian ; the adult irregulär, 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 hyssiferous ; Station ßuviatile. Pleistocene and Recent. 



The young shell of Bartlätia has well-niarked nymphae and internal resilium. The 

 relationship of the Naiadacea to Pteria renders the remarkable resemblance of the adult 

 Muelleria to Ostrea less surprising, siiice Ostrca is now known also to be derived from the 

 Pteriidae. 



Etheria Lam. Ostreiform, attached to rocks in African rivers. Also Pleistocene 

 of West Africa. 



Muelleria Ferussac ; Bartlettia Adams. Recent ; South American rivers. 



Superfamily 6. TRIGONIACEA Bronn. 



Shell equivalve, inequilateral, closed, dimyarian, not alate ; shell suhstance 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. Ordovician and Silurian. 



LyrodesmM Conr. {1 Actinodonta Phil.). Shell oval, cardinal boixler narrow, wiili- 

 out ligamentary area. Ordovician and Silurian ; America and Europe. 



