154 DR. J. Q. DE MAN ON THE PODOPHTHALMOTJS 



form of the clielipedes ; and finally tliat a gradual transition is 

 formed between them by the shape of the anterior legs of the 

 four other species. 



I may call attention to a short, horny, longitudinal crest, 

 characteristic of the male, and found on the upper (=inner) 

 surface of the arm of the chelipedes, lying parallel and close 

 to, or even on, the anterior margin of it, which I propose to 

 call the " musical crest." This crest is found in the genera 

 Metaplax and Helice, and in some species of Macrophthalmus 

 (e. g. in M. tomentosus, Eyd. & Soul.). I suppose that musical 

 sounds are produced by the crab rubbing this crest along 

 the row of lobules, teeth, or granules situated on the inferior 

 margin of the orbits, which I shall name " the infraorbital ridge." 



In all species of Metaplax and Helice the inferior margin of 

 the orbits is provided with lobules or teeth; these lobules or 

 teeth do not occupy the whole inferior margin of the orbits, 

 but the ridge on which they stand leaves the margin at a small 

 distance from the external angle. In the male this infraorbital 

 ridge is prolonged backwards, parallel to the lateral margin 

 of the cephalothorax, to some distance behind the orbits, so that 

 we may distinguish an " orbital portion " of the infraorbital 

 ridge from a " postorbital portion." In the female, however, 

 at least of those species which I have been able to examine, 

 the infraorbital ridge is never prolonged behind the orbits, and 

 its teeth or lobules are always smaller than in the male. 



The genus Metaplax belongs to the group of Sesaronacea in 

 the close vicinity of Helice, and it presents the following cha- 

 racters : — 



Cephalothorax broader than long, rather thin and little convex 

 above. Lateral margins toothed. Eront little deflexed, rather 

 narrow, measuring about a third or a fourth of the breadth of 

 the cephalothorax, with oblique lateral margins, and less promi- 

 nent than the epistome. External maxillipeds widely gaping, 

 with an oblique piliferous ridge ; merus-joint as long as, or 

 scarcely longer than broad, and about as large as the ischium- 

 joint. As regards the structure of the orbits, of the pterygo- 

 stomian regions, and of the inflected sides of the cephalothorax, 

 also as regards the shape of the sternum and of the abdomen, the 

 genus Metaplax perfectly agrees with Helice, de Haan, though 

 the pterygostomian regions and the inflected sides of the carapace 

 are generally less densely reticulate. 



