6 BULLETIN 168, UNITED STATES NATIONAL MUSEUM 



use would logically even separate individuals of the same species 

 (those of P. sepulcralis, for example, in which the ankylosis occurs 

 occasionally) into different genera, in some instances. 



Ankylosis of the last three abdominal segments, and the sub ventral 

 position of the anus, are the most important distinguishing characters 

 of the genus Folsomia, which in most respects is allied to Proisotoma. 

 Folsomia has affinities mth Isotomodes also; in the possession of 

 manubrial hooks, for example. 



Isotoma is essentially a natural genus, even though there are 

 intergrades between Isotoma and Proisotoma. The characters given 

 by Borner (1906) for separating the two genera hold good in all but 

 a few instances. 



The presence of bothriotricha need not exclude Isotomurus and 

 Axelsonia from the family Isotomidae, to which they evidently 

 belong by reason of all their other characters. 



Anurophorus, though lacking a furcula, is actually a very special- 

 ized form (Willem, 1900). 



Attention should be called to the primitive nature of Folsomides. 

 In this genus the body segmentation is of the simplest kind, the seg- 

 ments being essentially alike, relatively unmodified. Even the 

 third and fourth abdominal segments are simple rings. The fifth 

 and sixth are also simple and not greatly shortened. The furcula is 

 short. Shoidd it be regarded as reduced or not? If the ancestors of 

 Folsomides had a well-developed furcula, we would expect to find 

 remaining in Folsomides modifications of the posterior abdominal 

 segments, particularly the furcal segment, which are associated with 

 a strongly functional furcula. These segments, however, are still 

 pruuitive in form. It is possible, then, that the fm'cula in Folsomides, 

 instead of being reduced, actually represents an early stage in the 

 evolution of the organ. 



Folsomides recalls Tullbergia in its habitus, but it is placed in Iso- 

 tomidae chiefly on account of the nature of its postanteimal organ. 



Isotomodes is in many respects hke Folsomides, particularly as re- 

 gards the sensory organ of the third antennal segment, but it is more 

 specialized in having the genital and anal segments ankylosed and 

 the anus ventral in position. The manubrial hooks, strongly de- 

 veloped in Isotomodes, reappear moderately developed in Folsomia. 



The following genera of Isotomidae are not yet known to occur in 

 North America: Adaletes Giard (1889), Uzelia Absolon (1901), 

 Cryptopygus Willem (1902c), Proctostephanus Borner (1902b), Axel- 

 sonia Borner (1906), Colohurella Latzel (1917), Pseudanurophorus 

 Stach (1922a), Bornerella Denis (1924d), Spinisotoma Stach (1926), 

 and Astephanus Denis (1927). 



