1910.] BRITISH SPECIES OF OSTRACODA. 215 



of the valves must be eliminated, but in other respects the 

 generic definition is perfectly applicable. Whether the fact of 

 the three-clawed caudal ramus is sufficiently important to call for 

 separation from Herpetocypris may perhaps be doubted, but it 

 seems to be the only distinctive character, if we except the not 

 very important matter of shell-sculpture ; from Candona, on the 

 other hand, it is differentiated by the anatomical structures con- 

 nected with " parthenogenetic " propagation, as well as by the 

 presence, apparently, in the type species, of a setose fascicle on 

 the posterior antenna?. The two British species at present known 

 are Ilyodromus olivaceus and /. robertsoni. 



Ilyodromus robertsoni Brady & Norman. (Plate XXVIII. 

 figs. 10-15 ; Plate XXIX. fig. 12.) 



1889. Erpetocypris robertsoni Brady & Norman, (2) Part i. 

 p. 88. 



1896. Ilyodromus robertsoni Brady & Norman, (2) Part ii. 

 p. 724. 



Length P6 mm. Shell oblong, subovate ; seen laterally, oblong, 

 subreniform (PI. XXVIII. fig. 10), greatest height behind the 

 middle and equal to about half the length ; anterior extremity 

 broadly and rather obliquely rounded, posterior also rounded but 

 narrower than the anterior ; dorsal margin boldly arched, sloping 

 gently toward the point, more steeply backward, ventral margin 

 slightly sinuated : seen from above (fig. 11) the outline is com- 

 pressed, ovate, width much less than half the length, obtusely 

 pointed in front, rounded off behind. Shell-surface smooth ; 

 colour variable, but generally dark green with mottled patches of 

 lighter shades : left valve larger than the right and overlapping 

 distinctly at the two extremities ; the anterior margin rather 

 densely clothed with long hairs, ventral margin slightly hairy : 

 general surface of the shell structureless, but marked with 

 distant, very minute, circular punctures. The posterior antenna? 

 (fig. 12) are very sparingly provided with short seta? and have no 

 distinct fascicle ; palp of the posterior maxilla (PI. XXIX. fig. 12) 

 narrow and bearing three apical setaa ; two principal spines of the 

 first lobe of the anterior maxilla finely denticulated (PI. XXVIII. 

 fig. 15) ; terminal joint of the last leg (fig. 13) small, bearing a 

 slender falcate claw ; penultimate joint with a long seta near the 

 apex. Caudal rami (fig. 14) with three stout terminal claws pro- 

 gressively increasing in length towards the apex, a single slender 

 seta adjoining the last claw. 



The types of /. robertsoni were taken at Hayston Dam near 

 Peebles and at Portree, Isle of Skye. I have myself taken it in 

 a pool on Loughrigg, Westmorland, in a roadside pool near 

 Carrick, Co. Donegal, and in a ditch near Staward, Northumber- 

 land. It does not seem to have been observed by any Continental 

 authors. 



