PLEUROXUS. 135 



Abdomen (t. XVII, f. 3 c) gibbous for latter half, and 

 serrated. 



First pair of feet (f. 3 b} very large ; the pediform organ 

 of Midler being the first pair of feet. 



Intestine convoluted, having one turn and nearly half 

 another. 



Hab. Pond near Copenhagen Fields, Osterly Park, 

 Norwood Green, and Southall, Middlesex, June to 

 October. Pond at Fouldean, Berwickshire; and at 

 Yetholm, Roxburghshire. 



2. PLEUROXUS UNCINATUS. Tab. XVII, fig. 4. 



Shell slightly triangular in shape, transparent, and 

 fluted; gibbous on upper portion of anterior edge, and 

 less straightly cut on lower portion of anterior edge 

 than in preceding species. At the inferior angle of this 

 margin there are three sharp spines, and the margin itself 

 is ciliated. The posterior margin is rounded and sinuated 

 at the lower part, but not so deeply as in trigondlus, 

 and the square point at extremity is shorter than in it. 



The beak is long and curved ; but the tip, instead of 

 being curved downwards as in preceding species, is 

 turned up. 



The antennules, antennse, feet, and abdomen, with in- 

 testine, are similar to those parts in trigonellus, and the 

 animal is nearly of the same size. 



The ova, in all the specimens I have met with, are only 

 two. 



The young exhibit the remarkable turned-up beak and 

 the spines on inferior extremity of shell as soon as born. 



Hab. Pond between Hanwell and Southall, Middlesex, 

 September 1849. 



