238 



L. A. BORRADAILE. 



Morphologically, the Xanthids are characterised, among the other round-fronted crabs (Cy- 

 clometopa), by the absence of a rostrum, the sloping or transverse first antennae and short 

 second antennae, the sharp fore edge to the mouth, and the legs of the last pair not adapted 

 for swimming, nor the branchial regions swollen. Bionomically they are harder to characterise, 

 but, speaking broadly, we may say that they are not sand crabs, nor swimming crabs, nor 

 land crabs, but essentially crabs of the reef surface and the coral stocks, though some of them 

 may be found in other positions. Their division into subfamilies and genera depends on such 

 characters as the shape of the orbits and antennae, and the relationships of these to one 

 another, the ridges on the palate (endostome) which mark out the channels by which water 

 leaves the gill-chamber, and the shape of the body'. The species are separated by details 

 such as the shape of the front and the hands, the sculpture and areolation of the carapace, 

 and the presence or absence of spines on the limbs or the sides of the body. 



The body of a typical Xanthid — a Xantho, for instance (Fig. 50), or an Actaea (Figs. 4U., 

 53, 54), or Carpilius (Fig. 41 B), is of a transversely oval shape, heavy and compact, with a 

 hard cuticle and short legs, which can be folded up close under the body". The chelae are 

 usually large and powerful, and the habits sluggish. Such crabs live almost anywhere on 

 the reef sometimes showing a preference for some particular kind of shelter, such as holes 

 or cracks in the rocks, but more often, I think, adapting themselves to any that can be 

 found. Specimens are thus often taken in coral stocks, though the crabs cannot be said to 

 be characteristically coral crabs, being equally, if not more often, found under stones, which 

 afford the most favourite hiding-place of all ; some of the larger species are restricted to 

 the reef, but many of the smaller ones are also to be found in the lagoon wherever coral 



' lu the following key the sub-families of the Xanthidae 

 are defined by means of these features : 



I. Endostome ridges wanting. Shape of the body trans- 

 versely oval or rounded. 



A. Flagellum of antenna not shut out of orbital gap. 



1. Second joint of anteunal stalk cylindrical, reaching 



the front but not entering the orbital gap. 

 Xanthiiuie. 



2. Second joint of antennal stalk as in Xanthinae but 



entering the orbital gap. 

 Cdrpiliiiae. 



B. Flagellum of antenna shut out of orbital gap by part 



of the second joint of the stalk. 

 Etixinae. 



n. Endostome ridges present. Shape of the body often 

 square or squarish. 



A. Front less than half, and fronto-orbital edge (front 

 and orbits together) not more than two-thirds the 

 greatest breadth of the carapace. Front usually 

 makes an arch with the anterolateral edge. Fla- 

 gellum of antenna usually not phut out of orbital 

 gap. 



1. Second joint of antennal stalk cylindrical, and may 

 or may not reach the front, with which it is not 



broadly in contact. Endostome ridges vary in 

 shape and size. 



3Ie)iippinac. 



2. Second joint of antennal stalk somewhat irregular 

 in shape and broadly in contact with the front. 

 Endostome ridges strong, and project on the fore 

 edge of the mouth. 



Oziinae. 



B. Front at least half, and fronto-orbital edge more 

 than two-thirds the greatest breadth of the cara- 

 pace. Front makes an angle with the antero- 

 lateral edge. Flagellum of antenna alwaj's shut 

 out of orbital gap. 



1. Endostome ridges strong, and project on the fore 



edge of the mouth. 



Eiiphiijiae. 



2. Endostome ridges moderate, and make no projection 



on the fore edge of the mouth. 

 Trapeziiime. 



The genera will be found ranged under these sub-families 

 below. 



- The figure (41 c) of Euxanthus exsculptus gives an excel- 

 lent example of the way in which the limbs are often folded 

 up so as to form with the bodj' a compact mass. Fig. 47 

 shows how the legs are sometimes specially shaped for folding 

 into a small compass. 



