296 A. Alcock — New Species of Oxyrhynch Crab. [No. 2, 



exception of those belonging to the Regular system, ought to polarise 

 light. Bat this could not be verified in the case of crystals opaque to 

 light. There is no such difficulty with electric rays, for all crystals are 

 transparent to them. As a matter of fact, all the above experiments 

 with one exception were performed with specimens opaque to light. 



Explanation of the plate 



R... metallic box containing the Ruhmkorff's coil. 

 S... position of the sparking balls. 

 L... position of the convex Lens. 

 P...the Polariser. 

 I... Iris diaphragm. 

 K... the Crystal. 

 A... the Analyser. 

 C ...the Coherer. 



G...the Galvanometer. In practice the Galvanometer is placed 

 at a greater distance and the leading wires enclosed in 

 tin-foil. 



Description of a New Species of Oxyrhynch Crab of the Genus Parthenope. 

 —By A. Alcock, M. B., C. M. Z. S., Superintendent of 

 the Indian Museum. 



[Read 3rd July.] 



The species here described is a true Parthenope as delimited by 

 Miers, Journ. Linn. Soc, Zool., Vol. XIV. 1879, p. 668. 



Parthenope investigatoris, n. sp. 



Carapace almost equilaterally triangular, the sides very slightly 

 curved : its surface is deeply eroded and rugose as in P. horrida and 

 spinosissima, but is almost devoid of the sharp tubercles found in 

 those species : the antero-lateral borders are slightly crenulate : the 

 produced postero-lateral angle is rounded and nearly smooth : the 



posterior border bears five small eroded lobules a very small one 



in the middle line, with two larger ones on either side with 



intervening granules. The gastric region is enormously inflated 

 as in P. spinosissima, and descends almost vertically to the vertically 

 deflexed rostrum, the latter being fused with the interantennulary 



