14 BULLETIN 168, UNITED STATES NATIONAL MUSEUM 



ankylosis; fourth segment slightly longer than the third; genital and 

 anal segments relatively unmodified. Anus caudal. Head progna- 

 thous, mandibulate; mandibles with well-developed molar surface. 

 Eyes reduced in number (either one or two on each side, in the known 

 species). Postantennal organs present, relatively large, elongate, 

 narrow, straight to feebly curving, with thick wall. Antennae arising 

 relatively far forward, 4-segmented. Sense organ of third antennal 

 segment consisting of a pair of papillae, each situated in a pit. Fourth 

 antennal segment with slender curving olfactory setae. Unguiculus 

 present. Tenent hairs absent. Ventral tube emitting a pair of short 

 rounded vesicles. Furcula present, small, appended to the fourth 

 abdominal segment. Manubrium much longer than dens plus mucro; 

 manubrial hooks undeveloped. Dentes smooth dorsally, not crenulate 

 or tuberculate. Mucro bidentate, either separated from dens by a 

 suture or confluent with dens. Anal spines absent. Clothing of 

 short simple setae. Body pigment absent. 



Hitherto only a single specimen of this peculiar genus has been 

 laiown — the genotype, F. parvulus, found in Hungary and described 

 by Stach, to whose generic description I have added a little. 



Folsomides is evidently next to Isotomodes, in which, however, the 

 genital and anal segments are ankylosed, the anus is ventral, and the 

 manubrial hooks are strongly developed. Folsomides is unique among 

 Collembola in the primitive nature of the segmentation of the body. 

 The abdomen, with its simple, ringlike, equal segments is almost 

 vermiform; even the genital and anal segments are relatively unmodi- 

 fied, as compared with those segments in other Collembola. Both 

 Folsomides and Isotomodes show affinities with Folsomia and Proisotoma. 



KEY TO SPECIES OF FOLSOMIDES 



1. Two eyes on each side. Mucrodentes about one-half as long as 



manubrium parvus (p. 14) 



One eye on each side. Mucrodentes about two-thirds as long as 



manubrium stachi (p. 16) 



FOLSOMIDES PARVUS, new species 



Plate 2, Figures 16-20 



Description. — White. Greatly elongate (pi. 2, fig. 16); body sub- 

 cylindrical, six times as long as broad. Eyes (pi. 2, fig. 17) two on 

 each side ; equal, one in front of the other, separated, each on a small 

 irregular black pigment spot. Postantennal organ (pi. 2, fig. 17) 

 close to the anterior eye, narrowly elliptical or subelliptical, tliick- 

 walled, varying in length from a little shorter to a Uttle longer than 

 the width of the first antennal segment, with four guard setae. 

 Antennae inserted forward on the head, two-thirds as long as the 

 latter, with segments in relative lengths about as 2:3:3:5; fourth 



