782 Transactions of the Society. 



from the shell, and are much like those of Polyphemus, but longer. 

 The first pair have three joints, the first of which is long, and has 

 several short setce on its posterior margin ; near the base of this joint 

 is a two-jointed palp with short hairs at its end. The second 

 joint is shorter and carries two setae near its extremity ; the third 

 joint is longer and at its end has three or four long jointed setae. 

 The second and third pairs of legs are similar in structure to the 

 first, they are shorter and broader, each carrying an unjointed palp 

 on its basal segment. The fourth pair is short and two-jointed, 

 the second joint carries setae which radiate round it. 



There are two known species of Bythotre^hes, B. Gederstromii 

 and B. longimanus. The latter has never yet been found in 

 England. These two species, together with Polyphemus, Pleopis, 

 and Evadne, form the family Polyphemidae. 



Distribution. — I found B. Cederstromii first in Grasmere, but 

 subsequently in most of the large lakes in Cumberland and West- 

 moreland. It lives in the middle of large pieces of water and 

 seems to be more abundant in the autumn than the spring. 



Latona setifera Straus (pi. XII. figs. 9-14). 



Latona setifera presents several distinctive features from other 

 Cladocera. It is at once remarkable whilst swimming in the water 

 for its rectangular appearance, as seen in fig. 9. Its head is 

 large, and seen laterally (fig. 10) it appears, when compared with 

 those of other Cladocera, to be tilted backwards. The eye is 

 situated at the back, and the brain, which is small, in front ; the 

 optic nerves therefore run almost horizontally instead of vertically. 

 The eye consists of a pigment-mass surrounded by a number of 

 lenses which have a slightly reddish tinge. The pigment-spot is 

 small and lies near the brain. 



The division between the head and body is well marked. The 

 bead has no rostrum, and as it is tilted backwards a large portion 

 in front would be left quite exposed were it not that from the 

 edge of the shell a tongue-like projection extends downwards and 

 covers in all the organs of the mouth. This is a fixed portion of 

 the head unprotected by the shell and is not present in any other 

 known Cladocera. 



The shell valves are closely fringed with short setae along their 

 free margins, whilst at their anterior and posterior angles there are 

 bunches of unusually long setae ; it is from these that the name of 

 setifera has been given to the species. Each seta of the posterior 

 groups springs from a little knob on the margin of the shell, and 

 they present a very beautiful appearance. 



