A REVISION OF THE BRITISH IDOTEIDiB. 739 



Seas, on the Atlantic coast of North America (Harger), West Indies (Richardson), 

 Brazil, Java, and Red Sea (Miers), and from New Zealand (Chilton). 



Remarks— This species is easily distinguished from all other members of the 

 genus by the characters exhibited in the antennules, antennae, cephalon, the meso- 

 somatic segments, in the tridentate character of the posterior margin of the last 

 metasomatic segment, and the form of the uropoda. 



Minor characters are to be found in the form of the first maxillae, the maxilli- 

 pedes, and the thoracic appendages. 



Young examples, measuring 2*5 mm. in length, found on the metasomatic 

 appendages beneath the uropoda, strongly resemble the parents, excepting in the 

 form of the antennules, antennae, and the terminal segment. 



The maxillipedes of this species are larger and better developed than in any 

 other British member of the genus. On the anterior border of the third joint of 

 the palp there is a deep groove (PL I, fig. 7) in which the thickened posterior 

 border of the fourth joint moves. 



In examining the antennae of this and the other species here enumerated, I have 

 found the shape of the terminal style exceedingly constant, and have therefore given 

 figures of it in all the species. 



The actual number of joints in the flagellum varies according to age. I have 

 endeavoured to figure those in what I consider to be fully adult specimens. In 

 a young specimen of I. baltica, measuring 2 "5 mm. in length, the flagellum was 

 composed of only three joints and a style. 



(2) Idotea pelagica, Leach. (PI. II, figs. 15-25.) 



Idotea pelagica, Leach, Trans. Linn. Soc. Lond., 1815, vol. xi, p. 365; Bate and Westwood, Brit. 



Sessile -eyed Crust., 1868, vol. ii, p. 384, figs.] Dollfus, Feuille des jeunes Nat., 1895, p. 8, 



fig. 23. 

 Idothea pelagica, G. 0. Sars, Crust. Norw., 1897, vol. ii, p. 81, pi. xxxiii. 

 Idotea pelagica, Norman, Ann. and Mag. Nat. Hist., 1904 (s. 7), vol. xiv, p. 442; Tattersall, Nord. Plank., 



1911, vol. vi, p. 220, figs. 88-93. 



Body comparatively short and stout. Cephalon (fig. 15) wider than long, anterior 

 margin slightly concave, posterior margin straight, with sinuous line in front. Eyes 

 comparatively large, rounded, situated anterior to the median transverse line. An- 

 tennulae (fig. 16) small, first joint widely expanded, second less so, and third ex- 

 panded distally. Antennae (figs. 17 and 18) short and stout, extending just beyond 

 the first segment of the mesosome, first three joints almost subequal, as also fourth 

 and fifth ; flagellum short, composed of eight joints, terminal style short, with three 

 or four stout setae. First maxillae (fig. 19), outer lobe terminating in seven stout 

 curved spines, a single fine-pointed one, and two toothed ones, inner lobe with three 

 setose spines. The segments of the mesosome (fig. 23) are about equal in length, 

 anterior angles of the pleural plates of the first segment bluntly pointed. Coxal 



