4 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 123 



bionts on the crayfish host, the crayfish was killed, dissected into 

 units, and the specimens from each unit separately preserved. 



Most of the crayfishes and ostracods on which this study is based 

 are in the collections of the U.S. National Museum; the branchiobdel- 

 lids are in the collection of Holt, but most of these will be deposited 

 in the same museum. 



Presentation of data. — Brief discussions of the habitat, popula- 

 tion sizes and fluctuations, and food habits of each of the three groups 

 precede the sections devoted to the individual species. Within the 

 discussions devoted to each group, brief accounts of the anatomical 

 features utilized in the identification of the species are followed by 

 illustrated keys and generic diagnoses. For each species, a diagnosis, 

 the range, the distribution in the area, and the drainage systems, 

 range of elevations, and associates are presented. Data relative to 

 associations are summarized in table 1, and a discussion of those 

 associations concludes the study. 



Acknowledgments. — We should like to express our appreciation 

 to the following persons who assisted us on one or more of our collecting 

 trips in the Mountain Lake region: Messrs. Paul Buhan, J. E. Carico, 

 C. J. Freeman, R. H. Gilpin, H. H. Hobbs, III, H. A. James, E. P. 

 McConnell, Dr. F. A. Marland, Mr. Homer Mumaw, Dr. John J. 

 Neal, Jr., Dr. Jean E. Pugh, Miss Lucile Walton, Mrs. Shirley Wells, 

 and Miss Mary Lou Wood. Also, we are grateful to Dr. Robert D. 

 Ross for his discussions of stream piracy and problems of distribution 

 of fishes in the area. 



The Habitats 



The crayfishes. — Of the six species and subspecies of crayfishes 

 in the area, four vie for the habitats provided by tributaries of the 

 James River (Hobbs, 1951). In the uppermost reaches of both 

 headwaters and lateral tributaries, almost without respect to altitude, 

 Cambarus b. bartonii is the sole inhabitant and utilizes cover provided 

 by rocks, roots of shoreline plants exposed in the water, and most 

 debris occurring within the stream bed. As the stream begins to 

 exhibit alternating riffles and pools, in contradistinction to cascading 

 areas and pools, C. I. longulus takes over the major portions of the 

 riffles while C. b. bartonii occupies the pools and takes advantage 

 of the rock cover along the margins of the riffles. Downstream, as 

 the alternating pool and riffle areas become more pronounced and 

 more extensive, C. acuminatum replaces C. b. bartonii in the pools and 

 is frequently abundant immediately above and below the riffles, 

 utilizing, for the most part, cover provided by the larger rocks. In 

 the areas where the three may be found, C. b. bartonii seeks the 

 marginal portions of both the riffles and pools and frequently con- 



