78 F. B. ISELY. 



Survey for the Bureau of Fisheries, I found a fairly good number 

 of species in the early juvenile period of development. The 

 distribution and ecology of these young mussels seem to me to 

 be of special interest. Furthermore, the facts in regard to their 

 habitat seem to clear up some ecological perplexities concerning 

 the adult ecology of the Unionidse. 



Thirty-two specimens are here especially considered, these 

 were attached to rocks and pebbles by a functional byssus. 

 These mussels, representing nine species, were found in three 

 different rivers and in four localities. The situations, however, 

 were similar. Twenty-nine of these specimens weigh less than 

 five decigrams; of this number twelve weigh under one decigram, 

 and ten are under nine millimeters in length. 



The thirty- two specimens represent the following species: 1 

 (i) Lampsilis luteola, two; (2) Lampsilis fallaciosa, one; (3) 

 Lampsilis parva, four; (4) Lampsilis gracilis, three; (5) Plagiola 

 elegans, one; (6) Plagiola donaciformis, sixteen; (7) Anodonta 

 imbecillis, two; (8) Ptychobranchus phaseolus, two; (9) unnamed 

 species, one. 



The first of these specimens was secured on August 20, 1910, 

 in the Kiamichi River, near Fort Towson, Oklahoma. While 

 working in a gravel bed, Owen Home, one of our party, called 

 attention to an unusually small mussel attached to pebbles by 

 a byssal thread. After a search for about two hours we secured 

 eight specimens with byssus, representing four species. These 

 specimens were found in a coarse gravel bed, the pebbles being 

 from one fourth to one inch in diameter. The water was fairly 

 swift, and from one to two feet in depth. The byssus of these 

 specimens was variable in length, sometimes several inches long, 

 and often connecting three or four small pebbles. Sometimes 

 the byssus spread into several branches at the place of attach- 

 ment. The young mussels were best secured by taking up hand- 

 fuls of gravel and looking for the thread. The byssus is strong 

 enough to support the mussel in a rapid current, and will sustain 

 the weight of a number of small pebbles, without breaking. 



On August 30, 19 io ; five more specimens were secured in the 



1 Most of the specimens have been examined by F. C. Baker and L. S. Frierson, 

 making sure of the identifications. 



