K0.i205. SYXOPSIS OF THE XAIADESSIMPSOy. 507 



10 Mouocondyla-a, South America. 

 4 Iheringella, South America. 

 2 Fossula. South America. 



53 Glabaris, South America. 



8 varieties. 

 8 Mycetopoda, South America. 



3 Leila, South America. 



Total: 61 genera of Uuiouida? ; 11 genera of Mutelid;i\ 



I am inclined to believe with von Ihering that the primitive beak 

 sculpture of the Uniouid;e was radial, aud in two species of Uniosfrom 

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

 of Texas, 1 which are probably the oldest forms known, the beaks clearly 

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

 are not in condition to exhibit this character. 



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

 organixed form of uiiionoid life. 1 believe that the earlier Uuios had 

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

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

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

 radially sculptured beaks, the Endobranehs, 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 ;ilone, the JBxobrancks. 



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 Unionidre 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 the primitive 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 Unionidie originated in ^orth 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 period. All the 

 species of that family in South America have radial beak sculpture 

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

 and the young are contained hi the inner gills alone, so far as we know. 

 In some cases this sculpture is strictly radial; more often we find 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 land bridge in the 

 Antarctic region it is probable that a migration took place from South 



1 Proc. I'. S. Nat. Mus., XVIII, 1896, pp. 381-385. 



