CLASS I PELECYPODA 455 



mussel {Mya margaritifem Linn.), iu which the iDosterior laminae of the hinge are 

 obsolete. It shows in the majority of species amor^^hous, heavy, radial, pseudocardinal 

 and lateral teeth on the hinge ; the shell is variable in form and ornamentation, some 

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

 are pearly, with a conspicuous Ijrown or greenish periostracum ; the anterior adductor 

 scars are high, and the j^edal scars conspicuous. 



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

 muddy water. Tertiary and Recent. 



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

 anatomically intermediate between the Unionidae and Mutelidae. Tertiary and Recent. 



Family 29. Mutelidae Gray. 



Shell resemhling that of the Unionidae, itrithout pseudocardinals and laterals ; 

 having, tohen dentiferous, an irregularly taxodont hinge armature ; beaks unsculptured ; 

 mantle lobes generally partly closed ; siphons more complete. Nepionic stage {knoivn 

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

 to Recent. 



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

 Africa, and doulitfully j^resent also in the Upper Cretaceous of Provence. 



Anodontites Bruguiere {Glabaris Gray) ; Leila Gray ; Monocondylaea and Mycetopoda 

 d'Orb.; Mutela Scopoli (Tridina 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, ivith 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 Bartlctlia has well-marked nympliae and internal resilium. The 

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

 Muellcria to Ostrea less surprising, since 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 substance nacreous 

 and prismaAic ; 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. 



Lyrodesma Conr. (? Actinodonta Phil). Shell oval, cardinal border narrow, with- 

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



