SYNOPSIS OF TSE XAIADES— SIMPSON. 507 



10 MonocondyliEa, Sotith America. 

 4 Iheringella, South America. 



2 Fossula. Soutli America. 



3 Leila, South. America. 



53 GlabariSj South America. 

 8 A'arieties. 

 8 Mycetopoda, South America. 



Total: 61 genera of Uniouidfe; 11 geuera of Mutelidie. 



I am inclined to believe with von Iheriug' that the primitive beak 

 sculpture of the Uniouidai was radial, and in two species of Unios from 

 what are believed to be Triassic or Permian strata of the Staked Plains 

 of Texas/ which are probably the oldest forms known, the beaks clearly 

 show strictly radial sculpture. Four other species trom the same lot 

 are not in condition to exhibit this character. 



Xow I take this to belong to the simplest, earliest, and most lowly 

 organized form of unionoid life, I believe that the earlier Unios had 

 the young contained in the inner branchiiie alone, that there has been 

 a gradual development from these primitive forms with simple, dull- 

 colored, smooth shells, tliose of the male and female being alike, with 

 radially sculx^tured beaks, the Undobranchs, up to the highest forms of 

 to day, with concentric, doubly looped beak sculpture, with highly 

 painted shells, in which those of the male and female are very different, 

 with the young contained in distinctly marked ovisacs in the hinder 

 part of the outer gills alone, the Uxohranchs. 



The data for following these developments and the migrations of the 

 Naiades are meager so far as fossil material is concerned. But, fortu- 

 nately, while among the higher orders of life genera and even families 

 appear, develop, grow old, and become extinct in a single geological 

 age, the Uuionidie have held on unbroken from the Triassic or prob- 

 ably an earlier geological age until^now, and while there has been slow 

 progress in the development of higher characters tlie jyrimitive forms 

 have not died out. I know of no important type of the family among 

 the fossil species that may not be found somewhere to-day among the 

 living ones. They seem to have migrated to a certain region, made a 

 slight advance over the characters of their predecessors, and to have 

 continued down with but little change until to-day. When a new 

 migration was made the same thing was enacted again. 



If the Unionida? originated in North America during the Triassic or 

 some earlier period we may suppose that some members of the family 

 migrated into South America during that or at a later i^eriod. All the 

 species of that family in South America have radial beak sculpture 

 (except Callonaia and Prisodou, in which the beaks seem to be smooth), 

 and the young are coiitained in the inner gills alone, so far as we know. 

 In some cases this sculpture is strictly radial; more often we iind the 

 central or all the bars curving a little toward each other below, and 

 one or two of the middle pairs coalescing, the first move toward concen- 

 tric beak sculpture. By an old, now partly submerged laud bridge in the 

 Antarctic region it is i^robable that a migration took i^lace from South 



3 Proc. U. S. Nat. Mus., XVIII, 1896, pp. 381-385. 



