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THE VOYAGE OF H.M.S. CHALLENGER. 
adult males) are usually of moderate size, sometimes rather slender and very considerably 
elongated ; the dactyli of the ambulatory legs are styliform and are not armed with 
strong spines as in the Geocarcinidse. 
The species are for the most part small and littoral or shallow-water forms, but 
occasionally occur in deep water. 
Subfamily 1 . Carcinoplacina;. 
Carapace transverse, usually convex, and more or less cancroid in form, with the 
antero-lateral margins arcuated, spinose, or dentated, rarely entire. Frontal region 
usually of moderate width, orbits rather small. The post-abdomen at the base usually 
covers the whole width of the sternum between the bases of the fifth ambulatory legs. 
The fifth joint of the exterior maxillipedes articulates at the antero-internal angle of the 
merus. Chelipedes in the adult male usually subequal, and sometimes considerably 
elongated. Ambulatory legs moderately elongated, slender, with the dactyli styliform, 
sometimes compressed. 
The genus Pseudorhombila must be taken as the type of this family, since in it the 
characters are most evident, and it is the only genus included in the Histoire Naturelle 
cles Crustaces; moreover the designation has priority over Ccircinoplax, which is the 
name proposed by Milne Edwards in 1852, for the species of Curtonotus , de Haan. 
Curtonotus having been previously used in the Coleoptera. 
The genera have been divided into the following sections, which are apparently con- 
nected by insensible gradations : — 
1. Euryplacinse. 
Euryplacinaz, Stimpson, Bull. Mus. Comp. Zool., vol. ii. p. 150, 1870. 
Eucratopsinse , Stimpson, tom. cit., vol. ii. p. 151, 1870. 
In this section the antero-lateral margins of the carapace are dentated or spinose, and the 
post-abdomen in the male does not entirely cover the sternum at the base. In the typical 
genera, which are nearly related to the Cancroidea through Pctnopeus and Galene, the 
last segment of the sternum is exposed at the anterior corners only ; these genera are, 
Euryplax, Stimpson, Panoplax, Stimpson, Eucratopsis, S. J. Smith, and perhaps Glypto- 
plax, Smith (this genus is placed by Milne Edwards in the Cancroidea near Panopeus). 
In other genera, e.g., Speocarcinus, Stimpson, Eucratoplax, A. Milne Edwards, and 
Prionoplaoc, Milne Edwards (if this genus truly belongs here), the post-abdomen of the 
male is much narrower at the base, and a large part of the posterior segment of the 
sternum is exposed. 
The genera of this section, except perhaps the typical species of Prionoplax, are all 
American, and are not represented in the Challenger collection. 
