COLLEMBOLA OF THE FAMILY ISOTOMIDAE 43 



are present, and six subapical setae arranged in a circle around the 

 dens; ventral surface naked except for a pair of subapical setae. 

 Mucro (pi. 14, figs. 143, 144) one-third or one-half as long as dens, 

 stout, bidentate, bilamellate ; the two lamellae extending from the 

 base of the mucro to the anteapical tooth; apical tooth small, up- 

 turned; anteapical tooth larger, suberect or slightly hooked. Rami 

 of tenaculum (pi. 14, fig. 145) tridentate; corpus with one stout curv- 

 ing ventral seta. General clothing (pi. 14, fig. 146) of short dense 

 curving simple setae, becoming a little longer toward the end of the 

 abdomen. Long outstanding sensory setae apparently absent. 

 Integument smooth. Length, 1.3 mm. 



The apical filament of the unguiculus is long in some individuals 

 and short in others. 



Cotypes.—V.S.ISIM. no. 42982. 



Distribution. — Recorded as follows: 



Massachusetts: Arlington, April 8, May 1, in an old pile of horse manure, a.nr' 

 under boards near by. 



Subgenus Proisotoma Borner 

 Proisotoma Borner, 1901a, p. 134. 



Subgenotype. — Isotoma minuta Tullberg. 



KEY TO SPECIES OP SUBGENUS PROISOTOMA 



1. Dentes dorsally smooth or with tubercles. Furcula attaining 



the ventral tube. Third abdominal segment a little longer 

 than the fourth or subequal to it. Unguiculus with a pair of 



inner lamellae. Tenant hairs absent _2 



Dentes dorsally with transverse folds. Furcula rarely attaining 

 the ventral tube. Third abdominal segment shorter than the 

 fourth „ _4 



2. Dentes subcylindrical, apicaUy rounded, dorsally smooth, sub- 



equal to, or only slightly longer than, the manubrium. Man- 

 ubrium with many ventral setae. Mucrones complexly lam- 

 ellate, 3-toothed. Third abdominal segment slightly longer 



than the fourth. Purple to brown schafferi (p. 46) 



Dentes narrowing more or less, not rounded apically, dorsally 



coarsely tuberculate, much longer than manubrium 3 



3. Mucrones complexly lamellate, 5-toothed. Manubrium more 



than half as long as dentes, with three ventral subapical 

 setae. Third and fourth abdominal segments subequal in 



length. Olive-green rainieri (p. 45) 



Mucrones not lamellate, 4- (or 5-) toothed. Manubrium half 

 as long as dentes, with many ventral setae. Third abdominal 

 segment longer than the fourth. Dark blue commujia (p. 48) 



4. Dentes usually shorter than manubrium, the two rarely sub- 



equal, stout, with relatively few transverse dorsal folds (up 

 to 10 or 12, rarely as many as 20), which are usually coarse. 

 Mucro with three teeth (four in bulbosa), lamellate in only 

 two species 5 



