394 THE VOYAGE OF H.M.S. CHALLENGER. 



Milne-Edwards (Sergestes frisii, Kroyer), but differs in the relative measurements of 

 several parts. The peduncle of the first pair of antennae is according to Kroyer nearly a 

 seventh (" septima ferme parte") shorter than the carapace in Sergestes atlanticus, and 

 a third shorter (" tertia parte brevior") in Sergestes arcticus. 



The peduncle of the second pair of antennae has the terminal joint in Sergestes 

 atlanticus scarcely the sixth of the length of the scaphocerite, while in Sergestes arcticus 

 it nearly equals the third part. 



The sixth somite of the pleon in Sergestes atlanticus is "scarcely the sixth of the 

 length of the animal," and much shorter than the length of the fourth and fifth somites 

 united, and about one-fourth longer than the telson. In Sergestes arcticus the sixth 

 somite of the pleon is about " one-fifth the length " of the animal, surpassing the length 

 of the fourth and fifth somites united, and nearly twice the length of the telson. 



A careful and close analysis of the species named by Milne-Edwards Sergestes 

 atlanticus and by Kroyer Sergestes frisii and Sergestes arcticus, has induced me to 

 place them as synonyms of one and the same species. I have in the preceding paragraph 

 given the points of difference which Kroyer takes to be of sufficient importance to 

 justify their being considered separate species. If one compares Kroyer's figure with 

 that of Milne-Edwards which was published so far back as March 1830, it will be seen 

 the differences are not very important, and I feel certain that I shall receive the 

 approval of future observers for uniting them. Kroyer's specimen, Sergestes frisii, was 

 about 13 mm. in length, and Milne-Edwards' specimen of Sergestes atlanticus was 

 30 mm. long. 



Mr. Sidney Smith gives a species of Sergestes arcticus as being 90 mm. in length. 



The largest of the specimens of Sergestes atlanticus. from a dozen different places, 

 in the Challenger collection is 50 mm., and the smallest perfectly developed male 

 specimen in the collection is 24 mm. 



If the animal so varies in length after it has attained adult sexual features, I think we 

 may also assume that with successive moultings in time a greater or less variation may 

 take place in the proportion of parts in relation to each other, for if the relative value 

 of one part to the others be constant there could scarcely be room for the origination of 

 species. 



Sergestes dorsispinalis, n. sp. (PI. LXXII. fig. l). 



Carapace rather more than one-third of the length of the animal. Rostrum short, 

 sharp-pointed, and horizontal. Cervical suture well developed, and situated halfway 

 between the frontal and posterior margins ; just anterior to the suture, in the median 

 dorsal line, is a small, anteriorly directed tooth. 



Pleon with the four anterior somites dorsally smooth and subequal in size; the 



