344 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 



Idotea irrorata — Continued. 



Harger, This Report, part i, p. 569 (275), pi. v, fig. 23, 1874 ; Proc. U. S. Nat. 



Mus., 1879, vol. ii, p. 160, 1879. 

 Verrill, Am. Joiir. Sci.,III, vol. vii, pp. 131, 135, 1874 ; Proc. Amer. Assoc, 



1873, pp. 309, 371, 373, 1874 ; This Report, part i, p. 310 (22), 1874. 

 Whiteaves, Am. Jour. Sci., Ill, vol. vii, p. 217, 1874; Further Deep-sea 

 Dredging, Gulf of St. Lawrence, p. 15, " 1874." 

 Idotliea tridentata Rathke, Fauna Norw.,Nov. Act. Acad., B. xx, p. 21, 1843 (J. 

 tridentata Latreille?). 

 Grubo, Ausflug uach Triest, p. 126, 1861. 

 f Idotea tricHspis Dekay, Zool. New York, Crust., p. 42, pi. 9, fig. 35, 1844. 

 Oniscm Balthicuii {Ideotea marina) Dalyell, Powers of the Creator, vol. 1, p. 228, 



pi. Ixiii, figs. 5-9, 1851 (O. Balthiciis FaUas^). 

 Oniscus {Ideotea) entomon Dalyell, op. cit. vol. i, p. 229, pl. Ixiii, fig. 10, 1851 



{not 0. entomon Linn^.). 

 Idothea pelayica, M. Sars, Chr. Vid. Selsk. Forh., 1858, p. 151, 1859 (too< of Leach). 

 "Idotea acuminata Eichwald, Fauna Caspio-Caucasia, p. 232-233, tab. xxxvii, fig. 



6,1842" (Czerniavski). 

 Idothea baUhica Meinert, Crust. Isop. Amj)h. Dec. Danise, pp. 21, 228, etc., 

 "1877" {Oniscus Balthicus Pallas?). 



Plate V, Figs. 24-26. 



Adults of this species are at once distinguished from the other species 

 of the genus on our coast by the tridentate abdomen, or pleon, and young 

 individuals, which often resemble I. phosphorea, may be distinguished by 

 the epimeral sutures, which exte.nd quite across the second and succeed- 

 ing thoracic segments. For character separating them from the other 

 Isopoda of the coast, see at the close of the generic description. 



The body is smooth, not tubercular nor roughened. The head is 

 nearly square, narrowing but slightly behind. The eyes are small. The 

 antennulte (pl. V, fig. 25 a) are short, hardly surpassing the third segment 

 of the antennsB. The flagellum of the autennfe (pl. V, fig. 25 h) is longer 

 than the peduncle, distinctly articulated, slender, and composed of from 

 twelve to sixteen segments in the adults. When reflexed it reaches the 

 third thoracic segment. The external lamella (l) of the maxillipeds 

 (pl. V, fig. 26 a) is about twice as long as broad, and is obliquely 

 truncated. 



Thorax with the external margins, as seen from above, forming in the 

 adults, a pretty regular curved Une, the segments being marked by in- 

 cisions instead of by serratures as in the other species. In the second 

 and third, as well as in the posterior segments, this margin is formed 

 whoUy by the epimera. 



The first three segments of the pleon terminate in acute teeth at the 

 sides. The fourth, or last segment, has its lateral margins straight, and 

 is more or less tridentate at the tip, the middle tooth being much the 

 largest. In the operculum (pl. V, fig. 25 c) the basal plate is about three 

 times as long as the terminal one, which is broadly truncate at the 

 apex. The stylet (s) on the second pair of pleopods in the males (pl. V, 

 fig. 26 6) is usually shorter than, or, in smaller specimens, about as long 

 as the lamella to which it is attached, and is abruj)tly bent toward the 



