38 NORTH AMERICAN 



to characterize each known genus by describing and illustrating a 

 typical species. 



When the work on the nematode chapter was begun there was 

 hardly a single adequately described North American species, so 

 that the initial task was one of pure discovery. This was followed 

 by the work of research and definition, and this finally by that of 

 selection and presentation of typical species in a manner suitable 

 to a college textbook. 



The famous Philadelphian, Dr. Joseph Leidy, appears to be 

 the only naturalist who had previously attempted to describe any of 

 our fresh water nematodes, and even he did but little. I have at- 

 tempted to rehabilitate his Anguilhda longa, which, as Bastian sur- 

 mised, belongs to the more modern genus Trilobus. I have had to 

 assume that the variety Leidy had before him was that most com- 

 mon in the ditches, brooks and rivers of the Chesapeake region, 

 and have given to this form the name Trilobus longus (Leidy) 

 Bastian. 



All the other species proved to be new, some of them repre- 

 sentatives of new genera. Most of the new genera have already 

 been described in my contribution to the Proceedings of the Wash- 

 ington Academy of Sciences, October, 1913. 



An aquatic species as here defined is any species inhabiting 

 either fresh water, or non-brackish swampy soil below the water 

 table ; hence a species that will not drown in fresh water ; a species 

 fitted to utilize oxygen dissolved in fresh water. 



Including those of the present article, the fresh water North 

 American species so far described are as follows : 

 Achromadora minima Cyatholaimus truncatus 



Actinolaimus radiatus Diplogaster fictor, Bastian 



Alaimus simplex Dolichodorus heterocephalus 



Anonchus monhystera Dorylaimus fecundus 



Aphanolaimus minor Ethmolaimus americanus 



Aphanolaimus spirurus Iota octangulare 



Bastiana exilis Ironus americanus 



Cephalobus setosus Mesomermis virginiana 



Cephalobus subelongatus Microlaimus fluviatilis 



Chronogaster gracilis Monhystera sentiens 



Cryptonchus nudus Mononchus similis 



