318 THE PEARLY FRESH-WATER MUSSELS— SIMPSOX. vol.xviii. 



Gahillotia, Servain/ is typified by G. pseitdopsis, Locard, of Lake 

 Autioch, Syria. I do not know its position. 



Zairia, Eocliebrune,' proposed for Z. (Uscrepans^ Eochebruue, etc., 

 from the Congo. 



Coltotopteriim, Bourgiiignat, ' proposed for C.privclarum, Bourguignat, 

 is i)robably a form of Anodonta cygnea. The publications containing the 

 last tliree genera are not accessible to me. 



From the foregoing descriptions of genera, I am able to deduce a diag- 

 nosis of the family Unionidai, which I think will contain all the valid 

 genera heretofore described, and which will have to be, in our present 

 state of knowledge of the anatomy, founded largely on shell characters. 

 These, I think, when understood, are sufBciently distinct and constant 

 for use in separating the two families Unionidai and Mutelida^. The 

 force of this statement is added to when it is considered that the 

 arrangement I propose, which is founded so largely on shell characters, 

 fully agrees with what we know of the facts of geographical distribu- 

 tion, of the paleontology of the Naiades, and the classification of v. 

 Ihering, based on the characters of the embryos. 



Family UNIONID.E. 



Shell usually equivalve and inequilateral, smooth or variously sculp- 

 tured, angular or rounded, symphynote or non-symi)hynote, covered with 

 a thick epidermis, which may be green, brown, yellowish, black, rayed, 

 or variously painted; healcs usucdly sculpUtred with, conceutric ridges, 

 corrugations, chevron-shaped or radial patterns, or ])ustules, often show- 

 ing remains of the nuclear shell; ligament opisthodetic, well-developed, 

 external except when the shell is symphynote, Iiiterior nacreous; with 

 or without hinge teeth, hut showing vestiges of them in every genus; when 

 present ahrays schizodont and arranged as cardinals, laterals (pseudocar- 

 dinals and pseudolaterals), or both; adductor scars generally distinct, 

 the anterior commonly impressed ; pallial line simple and generally well 

 marked; prismatic border usually narroic and not conspicuous. 



Animal: Labial palpi almost alicays wider than long, having the iipper 

 2)arts of the posterior margins united; anal opening usually separated from 

 the superanal. Mantle either free or closed posteriorly to form a bran- 

 chial opening. Embryo a glochidium, the soft parts being inclosed in a 

 pouch-shaped bivalve shell, either with or without hooks, and borne in 

 the inner or outer, or in all four leaves of thebranchife, which are modifi;ed 

 to form a marsupium.^ 



iBull. Soc. Mai. France, VII, p. 296, 1890. 

 "Bull. Soc. Mai. France, III, pp. 1-14, pi. i, 1886. 

 8 Bourguignat, Lettres Malaculogiques, pp. 45-48, 1882. 



*In the above description I Lave italicized tbe most important characters, and those 

 which contrast most strongly with the same in the Mutelidie. 



