DECAPODA 395 



Key to the divisions of Brachyura here described: 



Oi Carapace more or loss triangular, being narrow in front. . . .1. Oxyriiynciia 



02 Carapace more or less circular, elliptical or rectangular, with a wide- 



arched or straight front margin. 



&i Carapace elliptical, with an arched front 2. CYCLOMEXorA 



&2 Carapace rectangular, with a straight front margin 3. Catometopa 



Division 1. OXYRHYNCHA.* 



Carapace usually narrow and triangular in front and broad and 

 rounded behind; 9 pairs of gills; first antennae longitudinally folded: 



3 families. 



Family MAIIDAE. 



Carapace narrow in front, rounded and broad in the middle, taper- 

 ing anteriorly to the promment rostrum, which may be bifurcate: about 

 34 Ameiican genera, all marine and littoral and occurring principally in 

 southern waters; some of the species have the habit of concealing them- 

 selves by placing on their backs with their pincers, algae, sponges, etc., 

 which often grow there. 



1. LiBiNiA Leach. Spider crabs. Eyes can be retracted into orbits, 

 which conceal them; rostrum bifurcate; body covered with a dense 

 growth of chitinous hairs which give it a furi^^ appearance; it may also 

 have growing on it hydroids, algae, worm 



tubes, etc., w'hich tend to conceal it : 6 species. 



L. emarginataf Leach (Fig. 629). Cara- 

 pace evenly rounded behind and w^ith 

 spines on the back, 9 of these being median 

 in position; legs long, the chelipeds in the 

 male being very long; length of carapace 7 

 cm.; breadth 6 cm.: Atlantic coast from 

 Maine to Florida, very common on mud ^'^' ^^^T^ftSnr^'^^'*''''* 

 flats and oyster beds. 



L. dubia Milne-Edwards. Like the above, but with fewer spines on 

 the back, 6 of w^hich are median: Cape Cod to Florida; less common than 

 L. emarginata. 



2. Hyas Leach. Carapace more or less triangular; rostrum bifur- 

 cate, the 2 branches converging, eyes in orbits but not completely con- 

 cealed: 3 species. 



H. coarctatus Leach. Toad crab. Lateral edges of carapace dilated 



* J^ee "Catalogue of the Crabs," etc., by Mary J. Rathbun, Proc. IT. S. Nat. Mus., 

 Vol. 1.5, p. 231, 1892 ; also Proc. U. S. Nat. Mus., Vol. 16, p. G.3, 1893. "Synopsis of 

 the Oxyrhynchous and Oxystomatous Crabs of North America," by Mary J. Rathbun, 

 Am. Nat., Vol. 34, 1900. 



t See "On the Anatomy of Libinia emarginata," by E. A. Andrews, Trans. Conn. 

 Acad., Vol. 6, p. 99, 1884. 



