c. d. soar on the genus hydrachna. 275 



Table for the Preliminary Identification of British Species 



of Hydrachna. 



1. Body skin with papillae (2). 



Body skin smooth without papillae (1. U. hvigata). 



2. Body colouring symmetrically arranged in. black and red 



(2. H. geographica.) 

 With eye-plate in one piece (3). 

 With two decided eye-plates (5). 

 With only narrow chitinous ridges (8). 

 With gland-plates well behind the eyes, and in some cases a 



small chitinous ridge or muscle attachment between the 



eyes and gland-plates (9). 



3. Eye-plate much broader than long (4). 



Eye-plate wedge-shaped between the eyes, extending back- 

 wards into two horn-like projections, longer than broad 

 (3. //. paludosa). 



4. Plate oblong with rounded corners, both anterior and pos- 



terior line being curved inwards (4. H. schneideri). 



Plate of irregular shape. Posterior edge curved slightly out- 

 wards, anterior edge curved very much outwards between 

 the eyes (5. H. scatata). 



Plate of irregular shape. Posterior edge curved inwards and 

 anterior edge outwards (G. //. cruenta). 



5. Papillae of skin rounded (6). 



Papillae of skin pointed or spine-shaped (7). 



6. Eye-plates ribbon-shaped, widened on the anterior margin. 



Both ends rounded and slightly curved inwards (7. H. con- 



jecta). 

 Eye-plates angular in form, the apex pointing inwards at the 



posterior ends. e Anterior portion rounded and projecting 



forward level with the eyes (8. H. dlssimilis). 

 Eye-plates broad, wedge-shaped at posterior ends, with a 



thick chitinous ridge on outer margin running to the 



posterior points (9. H. globosa). 



