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. Postantennal organs (fig. 53) near 

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

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

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

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

 two guard setae. 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). 

 Mesonotum almost covering the pronotum. Third abdominal segment a little 

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

 Unguis (fig. 54) 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 

 second tooth; second and third teeth dorsal, large, subequal, subcorneal, slightly 

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

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

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

 ventral seta 3 . General clothing of abundant short simple setae (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 Isptoma tricolor, from Waco, Texas, in the 

 Alut-eum 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 

 sent to me by MacGillivray. 



Isotoma palustris lives in moist places, and is especially abundant along the 

 edges 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 Europe, from Canada and the United States, Azores islands, 

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

 Nova Zembla, Spitzbergen, Bear island. 



Isotoma palustris var prasina Reuter. 



Plate 6, figs. 53-57. . 



Isotoma Stuxbergi var. prasina Reuter, 1891. 



Isotoma palustris var. prasina Schott, 1894. — Dalla Torre, 1895. — Reuter, 

 L895. MacGillivray, 1896.— Schaffer, 1896, 1898, 1900a, 1900b.— Poppe and 

 Schaffer. 1897.— Carl, 1899, 1901.— Carpenter and Evans, 1899.— Wahlgren, 

 189<)c, 1906b— Borner, 1901a.— Krausbauer, 1902.— Agren, 1903. 



Isotoma palustris var. pallida Schaffer, 1896. — Borner, 1901a. — Krausbauer, 

 1902. 



