54 



G. O. SARS. 



[No. 7. 



compressed, wanting dorsal denticles; supraanal angle distinct, 

 extremity transversally truncated; caudal claws shorter than in 

 female and arising from a small knob-like projection nearly in 

 the middle of the truncated extremity; secondary denticle dis- 

 tinct. Colour pale corneous turning to orange, dorsal part of 

 shell, in females with winter-eggs, dark bluish. Length of adult 

 female 0,50 mm, of male 0,34 mm. 



Bemarlcs. — This is the largest and finest of 3 new species 

 of Alona which I have succeeded in domesticating in my aqua- 

 ries, and I have much pleasure in naming it in honour of Mr. 

 Eobert Archer, to whom I am greatly indebted for his 

 kind assistance in providing me with material for continued ex- 

 periments in hatching Australian Entomostraca. The present 

 form is readily distinguished from the other known species, as 

 well by the peculiar sculpture of the shell as by the form and 

 armature of the tail. 



Description. — The length of the largest female specimens 

 observed reaches about half a millimeter. 



The carapace is much compressed and, as seen laterally 

 (PI. 6, fig. l) oblongo- quadrangular in form, with the posterior 

 extremity obtusely rounded and but little narrower than the an- 

 terior. The dorsal edge is evenly arched, somewhat more in 

 specimens with winter-eggs than in specimens with summer-eggs, 

 and unites the posterior edges of the valves without any distinct 

 intervening angle; nor are the infero-posteal corners distinctly 

 defined, but evenly rounded off. The inferior edges of the valves 

 are throughout the greater part of their length nearly straight; 

 only in the most anterior part do they slightly ascend to the 

 infero-anteal corners, which are likewise rounded off. The head 

 is quite immobile, hood-like and semierect, its dorsal edge form- 

 ing together with that of the carapace a continuous curve; in- 

 feriorly it terminates in a rather prominent acute rostrum, pro- 

 jecting a little below the inferior edges of the valves. 



The surface of the shell appears, when the animal is wholly 

 submerged in the water, quite smooth, without any visible sculp- 

 turing. But when floating on the surface, the side out of the 



