5G BRITISH COPEPODA. 



caudal segments about as long as broad, bearing three 

 stout marginal spines, and two plumose apical setae. 

 Length ^ 5 -th of an inch (1 mm.). Male unknown. 



The only known locality for this species is the river 

 Nene at Peterborough, where I took it sparingly in a 

 little weedy inlet. 



This, and the following species, C. Northumbricus, 

 constitute a link between the two genera Cantho- 

 camptus and Mesochra, the first foot being similar in 

 structure to the typical Canthocamptus, having both its 

 branches triarticulate, while the remaining pairs of 

 swimming-feet have the inner branch biarticulate 

 after the manner of Mesochra. It seems better, how- 

 ever, to enlarge the generic definition so as to include 

 these forms, rather than to erect a new genus for their 

 reception, which would indeed, if the same principle 

 were fully carried out, necessitate the establishment of 

 not one but several new genera. The point on which 

 I base the separation of Canthocamptus from Mesochra 

 is the structure of the first foot, leaving altogether 

 out of sight the number of joints of which the three 

 following pairs are made up. 



The most important specific characters of C. tri- 

 spinosus are found in the structure of the fifth pair of 

 feet, the very slender 9-jointed anterior antenna, and 

 the three spines of the caudal segments. 



