81 



viz., Iaera marina, it is best to adopt the name baltica proposed by Pallas, 

 which name undoubtedly refers to the, present species. The name under which 

 this form has more generally been recorded, is that of Idothea tricuspidata Des- 

 merest; but the Pallas' name ought certainly to be preferred as the much older 

 one. It is the largest of the Norwegian Idothea 3 , and easily recognizable in both 

 sexes by the form of the terminal segment of the metasome, which I have found 

 to be pretty constant even in very young specimens. As is the case with other 

 species of the genus, the form of the body in fully grown, ovigerous females 

 slightly differs from that in male specimens, in so far as the greatest width 

 occurs somewhat farther forward, or across the middle of the incubatory pouch. 

 The male is also considerably larger than the female, and has the coxal plates 

 comparatively broader. 



Occurrence. — The species occurs rather frequently along the whole Nor- 

 wegian coast, from the Christiania Fjord at least to Tromso, in comparatively 

 shallow water among alg£e. Though being more generally of a rather stationary 

 habit, slowly creeping about on the bottom, the animal is by no means devoid of 

 swimming power. At times it may even be seen to move rather rapidly through 

 the water. In doing so, the valvular uropoda are found to be widely separated, 

 so as to allow the free play of the anterior pairs of pleopoda, by the strokes of 

 which, the animal is propelled through the water. 



Distribution.— British Isles (Sp. Bate), Kattegat (Meinert), Baltic (Pallas), 

 Dutch coast (Hoek), coast of France (Bonnier), Mediterranean (Heller), Black Sea 

 (Gzerniawsky), Atlantic coast of North America (Harger). 



2. Idothea pelagica, Leach. 



(PI. XXXIII.) 



Idothea pelagica, Lench, Trans. Linn. Soc. Vol. XI, p. 36 r >. 



S3'n: Idothea brencornis, Rathke. 

 „ — torosa, Raihke. 



Specific Characters. — Body comparatively short and stout, being scarcely 

 3 times as long as it is broad, with the segments very sharply marked off from 

 each other. Coxal plates in both sexes comparatively small, not contiguous. Me- 

 tasome scarcely more than half the length of the mesosome, terminal segment 

 rather broad, and rounded at the tip, with only a very slight indication of a 

 median prominence. Eyes comparativtly large, rounded. Superior antenna? with 

 the last joint of the peduncle but little longer than the 2nd. Inferior antenna? 

 unusually short and robust, scarcely reaching beyond the 1st segment of mesosome 



11 — Crustacea. 



