154 DR. J. G. DE MAN ON THE PODOPHTHALMOUS 
form of the chelipedes; and finally that 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 WL. tomentosus, Hyd. & 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 Metaplaw 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 Sesarmacea 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. Front 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. 
