D. J. SCOURFIELD ON AN ALONA AND A PLEUROXUS NEW TO BRITAIN. 73 



appeared to be armed with only seven swimming setae ; but as 

 the eighth seta (that on the first joint of the inner ramus), if 

 present, would probably be very short, it may have been over- 

 looked. 



Coming to the characteristic post-abdomen (Fig. 2), it will be 

 noticed that, although rather broad at the base, it tapers con- 

 siderably towards the extremity, and that there is no trace of 

 a posterior dorsal angle, the dorsal margin sweeping round in a 

 fairly bold, even curve to the point where the terminal claws 

 arise. The armature consists of seven or eight teeth, decreasing 

 in size anteriorly. Some of the teeth appeared to be notched or 

 split. Just above the teeth there is a line of exceedingly delicate 

 " scales," each consisting of a few parallel setae. 



Keilhack's description and drawing of the armature of the 

 post-abdomen differs somewhat from the foregoing, but not 

 sufficiently , I think, to preclude identification of the two forms. 

 Slight individual variations of the teeth on the post-abdomen are 

 not uncommon among the species, of Alona, and differences of 

 interpretation by observers working at these fine details of 

 structure under different conditions must also be taken into 

 consideration. 



The total length of the specimen figured was ^ in., that of 

 Keilhack's specimen -^ in. (0-5 mm.). A. costata is usually a 

 trifle more than this, say T V in. 



Pleuroxus denticulatus Birge. 



I obtained this species on August 30th, 1905, from a little 

 pond quite close to the railway station at Exminster, Devonshire. 

 Only a single specimen was seen, and at first I could not be quite 

 certain that it was really P. denticulatus, although it was 

 evidently nearer to that species than to any other. Professor 

 Birge, however, has now most kindly sent me examples of the 

 American form, and, after careful comparison, I can see no 



