1910.] BRITISH SPECIES OF OSfRACODA. 213 



what flattened arch, ventral gently sinuated in the middle : seen 

 dorsally it is compressed, subovate, widest in the middle, with equal 

 acuminate extremities. Length 2*7 mm. The posterior antenna 

 bears a fascicle of six very short setse, and its terminal claws are 

 slender and finely pectinated (fig. 6) ; the two principal claws of the 

 first maxillar segment are club-shaped (fig. 7) and have blunt, 

 terminal denticulations ; Last joint of the second pair of feet 

 forcipate, bearing at the apex a slender curved claw and one long 

 seta, the penultimate joint forming distally on its inner margin a 

 convex pad which is fringed with a series of curved bristles, on its 

 outer margin a small finger-like ciliated process (fig. 8). Caudal 

 rami slender, bearing just within the upper margin along the 

 greater part of their length an interrupted series of minute cilia ; 

 terminal claws unequal in length, delicately pectinated, the 

 marginal seta almost close to the apical claws (fig. 9). 



Not infrequent in Scotland ; less common in England. Noted 

 also by several Continental authors. 



Genus Prionocypris Brady & Norman. 

 Prionocypris serrata Norman. (Plate XXV. figs. 13-16.) 



1866. Cypris serrata Brady, (1) p. 371, pi. xxv. figs. 15-19, 

 pi. xxxvi. fig. 3. 



1896. Prionocypris serrata Brady & Norman, (2) p. 725. 



1900. Cypris serrata G. W. Midler, (5) p. 72, pi. xiv. figs. 3, 

 11, 14. 



1900. Prionocypris serrata Kaufmann, (4) p. 292, pi. xx. 

 figs. 10-12, pi. xxi. figs. 22-26. 



A very distinct and well-marked species, far different in shell- 

 characters from any other fresh-water form. There are also 

 several minor abnormalities in the structure of the contained 

 animal which seem to warrant the use of the generic name 

 proposed in 1896 by Brady and Norman (loc. cit.). The more 

 important distinctive characters are: (1) the extremely short 

 antennal seta? ; (2) the smoothly edged claws of the outer lobe of 

 the first maxilla?, and the dilated extremity of the palp ; (3) the 

 perfectly smooth margins of the very slender caudal rami, and the 

 position of the dorsal seta very near to the apical claws ; (4) the 

 simple termination of the second pair of feet which bear on the 

 small terminal joint only two seta?, one long, and one short and 

 claw-like. 



The species is nowhere common, but is widely distributed in 

 England. It does not seem to have been found in Scotland or 

 Ireland, but is noticed by several Continental authors. In addition 

 to localities already published I have taken it in a backwater of 

 the River Wye at Rowsley, Derbyshire, and Mr. Scourfield records 

 it from reservoirs at Walthamstow and Purfleet, Essex. 



The generic characters mentioned above, though none of them 

 separately very important, may, together with the exceptionally 

 marked characters of the shell, be taken as sufficient to warrant 

 the separation of the species as the type of a distinct genus. 



