56 BRITISH COPEPODA. 
caudal seements about as long as broad, bearing three 
stout marginal spines, and two plumose apical sete. 
Length 3th 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. Northwmbricus, 
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. 
