78 BULLETIN 168, UNITED STATES NATIONAL MUSEUM 



Abdominal botliriotriclia were not seen in tixe single specimen avail- 

 able, but may have dropped off. Length, 2.2 mm. 



(7o/?/pes.— U.S.N.M. no. 42997. 



Distribution. — Recorded as follows: 

 Canada: Arnprior, Ontario, October, C. Macnamara. 



Genus ISOTOMA Bourlet 



Isotoma Bourlet, 1839, p. 399 (part).— Borner, 1903, p. 139; 1906, p. 171.— 



LiNNANIEMI, 1912, p. 138. 



Desoria Agassiz, 1841, p. 384. — Nicolet, 1841, p. 57. 



Genotype. — Isotoma viridis Bourlet. 



Fourth urotergite usually shorter than the third; the two seldom 

 equal. Third urotergite ventrolaterally prolonged backward (pi. 38, 

 fig. 441). Paratergite of the fourth abdominal segment always united 

 to the tergite of the genital segment, or the two ankylosed without 

 suture. Wlien the furcula is extended, the membrane between the 

 paratergites of the third and fourth abdominal segments is exposed. 

 Genital and anal segments rarely ankylosed. Furcula long, almost 

 always attaining the ventral tube. Manubrium much shorter than 

 dentes, with many ventral setae. Dentes long, slender, tapering, with 

 very many dorsal transverse folds or crenulations. Mucrones triden- 

 tate or quadridentate, rarely lamellate. Tenaculum with many ven- 

 tral setae. Tibiotarsus without a distal subsegment. Tenent hairs 

 present or absent. Postantennal organ not notched or constricted. 

 Clothing of relatively long setae, the largest of which are not infre- 

 quently unilaterally serrate or branched. Bothi'iotricha absent. 

 Integument smooth. 



KEY TO SUBGENERA OF ISOTOMA 



1. Tibiotarsi with two or three tenent hairs 2 



Tibiotarsi without tenent hairs Isotoma (p. 87) 



2. Genital and anal segments not ankylosed. Furcula attaining 



the ventral tube or not. Dentes convergent Vertagopus (p. 81) 



Genital and anal segments ankylosed. Furcula attaining the 



ventral tube. Dentes not convergent Pseudisotoma (p. 78) 



Subgenus Pseudisotoma Handschin 



Pseudisotoma Handschin, 1924a, p. 111. 



Subgenotype. — Isotoma sensibilis Tullberg. 



ISOTOMA (PSEUDISOTOMA) SENSIBILIS Tullberg 



Plate 27, Figures 296-299 



Isotoma sensibilis Tullberg, 1876, p. 36. — Uzel, 1890, p. 04. — Schott, 1894, 

 p. 72; 1902, p. 28.— Reuter, 1895, p. 27.— Lie-Pettersen, 1896, p. 18; 

 1898, p. 13; 1907, p. 61.— Carl, 1899, p. 307; 1901, p. 264.— Carpenter 

 and Evans, 1899, p. 249.— Absolon, 1900a, p. 30.— Schaffer, 1900a, p. 

 247; 1900b, p. 258.— Wahlgren, 1900a, p. 367; 1906b, p. 257.— Evans, 



