COLLEMBOLA OF THE FAMILY ISOTOMIDAE 71 



The qualifications in parentheses apply to foreign species of the 

 genus. 



Only the bothriotricha and the type of mucro distinguish Isotomurus 

 from Isofoma. 



KEY TO SPECIES OF ISOTOMURUS 



1. Unguis extremely slender (pi. 2,5, fig. 277), without lateral teeth. 



Yellowsh green retardatus (p. 71) 



Unguis not conspicuously slender (pi. 26, fig. 285) and with a 



pair of lateral teeth 2 



2. Unguiculus setaceously prolonged. Clothing of short setae 



fringed on all sides. Violet palustroides (p. 77) 



Unguiculus not setaceously prolonged. Clothing of simple setae. 



Coloration variable palustris (p. 72) 



ISOTOMURUS RETARDATUS. new species 



Plate 25, Figures 276-280 



Description. — Yellowish green. Appendages pale green. Head 

 with two median dorsal roundish black spots; one anteocular, the 

 other postocular. Eyes (pi. 25, fig. 276) eight on each side, the two 

 inner proximal eyes of each side somewhat smaller than the others. 

 Postantennal organ (pi. 25, fig. 276) elliptical, slightly shorter to 

 one-tliird longer than the diameter of an adjacent eye. Antennae 

 longer than the head (as 7:5), with segments in relative lengths about 

 as 10:19:21:27; segments more or less blackish apically; third segment 

 subclavate; fourth elliptical, petiolate. Sense organ of tloird antennal 

 segment with a pair of subcylindrical rods subtended by a thick 

 chitinous ridge; fourth segment without special olfactor}^ setae. 

 Unguis (pi. 25, figs. 277, 278) exceptionally long and slender, uniformly 

 tapering and curving, untoothed. Unguiculus sublanceolate, acu- 

 minate, untoothed; on first and second pairs of feet extending one- 

 third as far as the unguis; on third pair, larger, extending more than 

 half as far as the unguis, aciculate. Tenent hairs absent. Third 

 abdominal segment equal to fourth or longer (as 7:6). Genital and 

 anal segments not ankylosed. Furcula appended to the fifth abdomi- 

 nal segment, attaining the ventral tube. Manubrium with dense 

 short stiff setae dorsally and ventrally. Dentes almost twice as long 

 as manubrium (as 7:4), slender, tapering, crenulate dorsally, with 

 numerous short stiff dorsal setae except on distal third, and dense 

 short stiff ventral setae. Mucro (pi. 25, fig. 279) quadridentate, 

 lamellate. Apical tooth of mucro small, at the base of the second 

 tooth, blunt or pointed, reduced or absent in some individuals; 

 second and third teeth large, in longitudinal alinement with the first; 

 fourth tooth proximolateral, oblique, blunt or acute; base of mucro 

 with a median dorsal rounded lobe. Outer and inner dorsal lamellae 

 extend from the base of the mucro to the second and third teeth, 



