REPTILIAN AGE JURASSIC PERIOD. 91 



arranged. The following are the names .of the genera most usually admitted by 

 late Avriters, viz. : Unio, Anodonta, Byssanodonta, Margaritana, Monocondyllcea, and 

 Barbala. Prof Agassiz, however, who has dissected, with much care, many of our 

 American species, finds that they present marked differences in the arrangement 

 of the gills, and the position of the eggs in the same, as well as in other anatomical 

 details, from which he is led to the conclusion that there are at least twenty-two 

 distinct genera amongst our species usually referred to Unio, Anodonta, Alasmo- 

 donta, and Margaritana. For these groups he has adopted the following names, 

 viz. : Dysnomia, Ag. ; Scalenaria (Eaf.), Ag. ; Truncilla (Eaf.), Ag. ; Lampsilis, 

 E.af. ; Canthyria, Swainson ; Eurynia, Eaf. ; Metaptera, Eaf. ; Alasmodonta, Say ; 

 Ohovaria, Eaf. ; Micromya, Ag. ; Cyprogenia, Ag. ; Plagiola, Eaf. ; Orihonymus, 

 Ag. ; Tritogonia, Ag. ; Quadrida, Eaf ; Botundaria, Eaf. ; Complanaria, Swainson ; 

 Pleurohema, Eaf. ; Uniopsis, Swainson ; Margaritana, Schumacher ; Hemilasterna, 

 Eaf, and Unio, Eetz.^ 



Mr. T. A. Conrad, of Philadelphia, who admits many of these groups as 

 subgenera under Unio, also proposes to adopt the following additional subgenera, 

 mainly for the reception of foreign species, viz. : Nodularia, Con. ; Iridea, Swainson; 

 Mysca, Turton; Lanceolaria, Con. ; Coelatura, Con. ; Cunicida (Sw.), Con.; Glebula, 

 Con. ; Uniomerus, Con., and Tlieliderma, Swainson. The following he proposes as 

 full genera : Cucumaria, Con. ; Hyriopsis, Con., and Monodontina, Con. 



As an example of the widely different views entertained by authors in regard to 

 the classification of these mollusks, we should remark that Dr. Isaac Lea, who has 

 given more attention to the study of the Unionidae than perhaps any other person, 

 includes the whole, along with some others not generally admitted in this family, 

 under two generic heads, for which he adopts the names Margaron and PJatiris. 

 Under the first of these he ranges as subgenera, Triquetra, Prisodon, Unio, Margari- 

 tana, Monocondylcea, Anodonta, and Dipsas, ; and under the second Iridina, SpatJia, 

 and Mycetopus} 



Mr. Niclin went still farther in this direction, and included Unio, Anodonta, Alas- 

 modonta, Iridina, Dipsas, Hyria, and Castalia as members of a single genus !^ 



The family Unionidcs has a wide geographical distribution, but is most numer- 

 ously represented in the streams of North America. Although apparently repre- 

 sented as far back as the Jurassic period, it is pre-eminently characteristic of the 

 present epoch, since the species and genera are far more numerous now than they 

 were during any of the past geological periods. The existing species also present 

 much greater diversities of form and ornamentation, and sometimes attain larger 

 sizes than are known to occur amongst those now extinct. 



* Weigmann's Arch. ,1852, p. 41. ' Synopsis Naides, 3d ed. Phila. 1852. 



» Trans. Phil. Soc. VIII, p. 398. 



