12 A Canadian Arctic Expedition, 1913-18 



shaped spot. Eyes 8 + 8 (fig. 53) subequal; or two inner proximal eyes of 

 each group a little smaller than the others. Postantcnnal organs (fig. 53) near 

 the eyes, elliptical, slightly longer than, to twice as long as, the diameter of an 

 adjacent eye. Antenna' once and one-half to twice as long as the head, with 

 >egmcnt> in relative lengths about as 3:4:5:6. Sense organ of third antennal 

 >egment with a pair of linear feebly curving sense rods, a thick basal ridge, and 

 two guard sehe. Very short curving sensory setae occur on all the antennal 

 segments as follows: segment 1, 2-5; 2, 3-7; 3, 3-7; 4, 10-15. On the first 

 three segments these are on the under side near the distal outer end; on the 

 fourth segment they occur on the distal half along the outer side (Agren, '02). 

 Mesonot uni almost covering the pronotum. Third abdominal segment a little 

 longer than the fourth (about as 5:4). Abdominal segments without ankylosis. 

 l"ngui> (fig. f>4) stout, curving, with a pair of small lateral teeth, and with inner 

 margin untoothed. Unguiculus broadly lanceolate, with inner lamella roundly 

 dilated basally, untoothed as a rule, extending a little beyond the middle of the 

 unguis. Tenent hairs absent, represented by a single long simple hair. Furcula 

 apparently appended to the fifth abdominal segment, and extending to the 

 anterior border of the ventral tube. Dentes twice as long as manubrium, 

 slender, gradually tapering, crenulate dorsally. Mucro two-thirds as long as 

 hind unguis, quadridentate (fig. 55). Apical tooth small, at the base of the 

 >econd tooth; second and third teeth dorsal, large, subequal, subconical, slightly 

 hooked, in longitudinal alinement; fourth tooth lateral, oblique, acute, extending 

 almost half the length of the mucro. Basal lateral mucronal seta present. 

 Kami of tenaculum quadridentate (fig. 56); corpus with many (fifteen or more) 

 ventral setto. General clothing of abundant short simple seta3 (fig. 57). Long 

 outstanding feathered sensory setae occur on the last five abdominal segments; 

 there being one or two pairs of these on each of the segments, and sometimes 

 three pairs on the fourth abdominal segment. Length, 3 mm. 



Packard's six cotypes of his Isotoma tricolor, from Waco, Texas, in the 

 MiiM-um of Comparative Zoology, Cambridge, Mass., for which MacGillivray 

 ('96, p. 48) retained the name of tricolor, I found to be palustris. In the same 

 tube with them were five specimens of Isotoma viridis, from Salem, Mass. 



/. aequalis MacG. is also palustris, as I have found from a study of a cotype 

 >ent to me by MacGillivray. 



Ixolmna paluxtris lives in moist places, and is especially abundant along the 



of ponds and streams, frequenting the vegetation in preference to the 



water, though it is at home on the surface of the water, where it leaps vigorously 



and repeatedly. The species occurs on the seashore also, under seaweed, drift- 



wood or stones, and is sometimes found on the snow. 



This is one of the dominant species of its order. It is cosmopolitan in 

 distribution, everywhere common, and highly variable in colouration, several 

 varieties having received names. The typical form of the species is known 

 from all parts of Kurope, from Canada aiid the Tinted States, Azores islands, 

 India, and .Java: the recorded Arctic distribution being as follows: Siberia, 

 Nova Zembla, Spit/bergen, Hear island. 



Isotoma palustris var prasina H< uter. 



I'l.-it.- <;, fin*. r^ r,7. 



Stu.rlxrt//' var. />/v/.v//m Renter, 1891. 



imlnstrix var. prasina Schbtt, 1894. Dalla Torre, IS!);"). Renter, 

 MacGillivray, 1890. Sch-iffer, 1890, 1898, 19()()a, 1900b. Poppe and 

 Schaffer, 1S97. Carl, 1S99. 1901. Carpenter and Kvans, 1899. \Vahlgren, 

 IVMir. I906b. Homer, I90!a. Kraiisbauer, 1902. Agren, 1903. 



^ Isotoma /iiihixlrix var. /i<tlli</<i S< haffer, 1896. Bonier, 1901a. Krausbauer, 



1 90_ . 



