98 



denticles, 2 apical and 2 lateral, that of the outer edge distinctly defined at 

 the base. 



Male more slender than female, with the anterior division of the body 

 less vaulted, and the urosome considerably longer and narrower. None of the 

 caudal setse dilated at the base. Right anterior antenna with the middle section 

 moderately tumefied, and exhibiting at the end anteriorly a serrated lamella. Last 

 pair of legs exhibiting the structure characteristic of the genus, left leg more than 

 twice as large as the right, with the 2nd joint greatly dilated and having the 

 thumb-like process long and slender, terminal joint shorter than the penultimate 

 one and scarcely broader, being produced at the tip to an acute projection, 

 outside which a ciliated seta is fixed, inner edge finely ciliated and carrying 

 2 small spines ; right leg with the terminal joint abruptly curved near the base, 

 and carrying outside a strong deflexed spine. 



Colour. Body semipellucid, with a faint bluish tinge, and generally 

 exhibiting on the anterior division some few, finely ramified pigmentary patches of 

 a light brown or reddish colour. 



Length of adult female 1.50 mm; of male about the same. 



Remarks. This Calanoid was described as early as in the year 1785 by 

 0. Fr. Miiller as Cyclops longicornis. It was, however, erroneously identified by 

 Baird and subsequent authors with Monoculu* fumarchicus of Gunnerus, which, 

 as shown above, is a true Calanus. The Diapiomus longicaudatus of Lubbock 

 seems to be the present species. Though easily recognizable from our other 

 Calanoids, this form exhibits great resemblance to some exotic species, especially 

 to Calanus turbinatus of Dana, and, indeed, Mr. Th. Scott considered it to be 

 the very same species. Dr. Giesbrecht, however, who seems to have had an 

 opportunity of examining both these forms, keeps them apart as distinct, though 

 very nearly allied species. 



Occurrence. This is one of our commonest Calanoids, being met with 

 along the whole Norwegian coast, both in the fjords and in the open sea. 

 Off the south and west coasts especially, it is often found in great abundance, 

 forming indeed an essential part of the zooplankton, and in all probability 

 therefore, having great significance as fish-food. In the Christiania Fjord I have 

 met with it during all seasons, even in the immediate neighbourhood of the town. 

 In habits it is a true pelagic form, occurring as a rule close to the surface of 

 the sea. It moves in a peculiar revolving manner, and this seems to be the 

 case with all the species of this genus, and may have given rise to the specific 

 name turUnata applied by Dana to one of them. 



