232 AMERICAN JOURNAL 



EEMAEKS ON THE GENERA MONOCONDYLCEA, 



D'ORB., AND PSEUDODON, GOULD, 'WITH 



A SYNOPSIS OF THE LATTER. 



BY T, A. CONEAD. 



The genus Monocondyloea, D'Orbigny, is composed of a 

 small group of Uiiionid£e whicli exclusively inliabits the waters 

 of South America. Several Asiatic forms have lately been 

 associated with them, which have a different external form 

 and character and a dissimilar hinge, excepting that they pos- 

 sess only one cardinal, and no lateral teeth, and might as well 

 be referred to Margaritana, Schumacher, as to Monocondylcea. 

 D'Orbigny remarks of the latter genus, that "it has an aspect 

 quite peculiar, by which it is immediately known." This, I 

 suppose, refers to its external aspect, which is sujEhcient to dis- 

 tinguish the genus from the other forms of Unionidae. It is one 

 of those peculiar South American groups, which form so 

 marked a feature of the Unionidas of that continent, as strictly 

 limited to it as Triquetra, Castalia, or Iridea, or as Pleiodon to 

 Africa. D'Orbigny's description of the hinge is as follows : — 

 " Dents cardinales non rentrante, formee d'un seul tubercle a 

 chaque valve, s'applicant I'un au-dessus de I'autre, celui de la 

 valve gauche etant audessus, celui de la valve droite au-des- 

 sous des crochets; le contraire existe quelquefois."* Dr. 

 Gould describes the Asiatic group under the name of Pseudodon, 

 with the following diagnosis: — "Hinge margin with a tooth- 

 like apophysis on each valve, the surface of which is not frac- 

 tured, but smoothly covered with enamel fitting into corre- 

 sponding undulations in the opposite valve, that of the right 

 valve closing in front of the left;" this last character is re- 

 versed in Monocondyloea^ the tooth of the left valve being 

 under the beak, and that of the right valve anterior to it. 

 This group is of an elliptical, trapezoidal or rhomboidal out- 

 line, wholly unlike the obtusely-ovate, ventricose shape of 

 Monocondyloea^ with its prominent beaks and excavated ante- 

 rior dorsal margin. 



Another distinct form is Monocondyloea Mardinensis, Lea, 

 from the Eiver Tigris. This has an outline approaching Mar- 



* This must be rare and exceptional, as in all D'Orbigny's figures of six 

 species the normal character is represented of the cardinal tooth being 

 situated under, or posterior to the apex. 



