70 BULLETIN 1G8, UNITED STATES NATIONAL MUSEUM 



convex ventral margin (in lateral aspect) and three approximate 

 teeth, the proximal tooth being lateral in position, weakly developed 

 and sometimes absent. Rami of tenaculmn quadridentate ; corpus 

 stout, with abundant (20 to 30) ventral setae. Clothing exclusively of 

 dense short stiff simple setae; a very long simple seta projecting from 

 the inner side of the femur. Integument smooth. Length, 2 mm 

 (Agren); 2.5 mm (Carpenter). 



In some individuals an obsolete dorsal suture occurs between the 

 genital and anal segments. 



Remarks. — The basal membrane of the unguis has been described 

 in detail by Denis (1924b), who holds that it is not the equivalent of 

 the tunica of certain Sminthuridae, but is an expansion of the lateral 

 teeth of the unguis. 



European examples of this species were kindly given to me by 

 Dr. C. Schaffer, Dr. E. Handschin, Dr. Jan Stach, and Dr. R. Denis. 



This species occurs under stones on the seashore, under partlj" 

 submerged stones in streams, and on the snow. It is distinctively an 

 arctic and alpine species (Carpenter) ; in arctic regions being abundant 

 and, in some places, the dominant species of its order. 



Distribution. — Agrenia bidenticulata has been recorded from Green- 

 land, Spitsbergen, Bear Island, King Charles Island, Franz Josef 

 Land, Nova Zembla, Siberia, Norway, Sweden, Finland, Lapland, 

 Russia, Switzerland, Scotland, and Ireland. 



Colorado: Ghost Glacier, August 31, G. W. Goldsmith. 



Wyoming: Yellowstone National Park, August 28, S. A. Forbes (Illinois State 



Natural History Survey). 

 British Columbia: Watershed of Franklin River, August 2, on surface of water 



and snow at 10,500 feet, Mr. and Mrs. D. Mundy. 



Genus ISOTOMURUS Borner 



Podura Muller, 1776, p. 183 (part). 



Isotoma BouRLET, 1839, p. 399 (part). — Tullberg, 1872, p. 44 (part). 



Isotomurus Borner, 1903, p. 171; 1906, p. 162. — Linnaniemi, 1912, p. 186. 



Genotype. — Podura palustris Muller. 



Fourth urotergite a little shorter than the third (subequal). Third 

 urotergite ventrolaterally prolonged backward. Paratergite of fourth 

 abdominal segment as in Isotoma. Genital and anal segments not 

 ankylosed. Furcula attaining the ventral tube. Manubrium about 

 one-half as long as dentes, with many ventral setae. Dentes long, 

 slender, tapering, and dorsally with very many transverse crenula- 

 tions. Mucrones quadridentate (tridentate), lamellate. Tenacu- 

 lum with many ventral setae. Tibiotarsus without a distal subseg- 

 ment. Tenent hairs absent. Clothing of relatively long setae, 

 sometimes fringed. Fringed abdominal bothriotricha present. In- 

 tegument smooth. 



