Ds PROCEEDINGS OF THE NATIONAL MUSEUM VOL, 125 
ACKNOWLEDGMENTS.—Part of this paper is based upon an un- 
published doctoral dissertation written at the University of Virginia 
(Holt, 1951). Some of the material studied was collected with the 
aid of grants from the National Science Foundation (NSF G—4439, 
G-9828, GB-372). The paper was prepared during the tenure of a 
Visiting Research Associateship at the Smithsonian Institution. As 
always, I am grateful to Dr. Horton H. Hobbs, Jr., for his interest 
in and support of my studies in branchiobdellid systematics, for a 
careful and helpful reading of the manuscript of this paper, and for 
identifying the host crayfishes. I wish to thank a number of collectors: 
Dr. Hobbs; Drs. Warwick R. West, Denton W. Crocker, and Joseph 
F. Fitzpatrick, Jr.; Messrs. C. W. Hart, Jr., and Kenneth W. Simonds; 
Mrs. Virgie F. Holt. 
METHODS AND DISPOSITION OF MATERIAL.—The material used in 
this study that was collected by persons other than Mrs. Holt and 
myself was collected and preserved in 70% ethyl alcohol. My collect- 
ing methods and procedures have been described elsewhere (Holt, 
1960a, p. 57). Except for some paratypes of new species and a small 
number of specimens retained for reference purposes in my collections 
at the Virginia Polytechnic Institute, the material is deposited in 
the collections of the United States National Museum. Where appro- 
priate, United States National Museum catalog numbers (USNM) 
and my personal catalog numbers (PCH) are listed with the locality 
data given for a species. In all cases, complete locality data are avail- 
able from my files or from the Registrar, United States National 
Museum. The terminology used in branchiobdellid taxonomy and 
evaluation of the taxonomic utility of various characters have also 
been discussed previously and the reader is referred to these papers 
(Holt, 1953, 1960a, Holt and Hoffman, 1959), but some relevant 
explanations and anatomical comparisons are presented, where appro- 
priate, in discussions of certain species. 
REVIEW OF THE LITERATURE.—Knowledge of the genus Pterodrilus 
dates from Moore’s paper entitled “Pterodrilus, a remarkable discod- 
rilid’’ (Moore, 18952). He separated the genus from previously known 
branchiobdellids (=“‘discodrilids” of authors, e.g., Vejdovsky, 1884) 
on the basis of the striking dorsal “appendages” of the two species 
(P. alcicornus and P. distichus) that he described and assigned to his 
new genus. His descriptions are excellent for the state of knowledge of 
the branchiobdellids of his time, and his species are easily recognized. 
Since then, Pierantoni (1912, pp. 24-25) and Stephenson (1930, p. 801) 
mentioned the genus in their literature survey. Ellis (1918, pp. 49-51), 
by means of a key, assigned his species durbini to Pterodrilus; sub- 
sequently (1919, pp. 254-255), he formally described and ilustrated 
P. durbini, described P. mexicanus, and listed new locality records for 
