THE SPIDER CRABS OF AMERICA 463 



suggests that a similar transformation may occur between young 

 S. triangulata and adult S. macdonaldi; but until this is proved by 

 intermediate, half-grown individuals, it is best not to assume that 

 the rostrum and orbits can so change with age as shown by com- 

 parison of plate 266, figure 1, and plate 268. 



Genus MACROCOELOMA Miers 



Pericera (part) Milne Edwards, Hist. Nat. Crust., vol. 1, 1834, p. 334. 

 Macrocoeloma Miers, Journ. Linn. Soc. London, vol. 14, 1879, p. 665; type, 

 M. trispinosum (Latreille). 



Carapace subpyiiform or suboblong, but broadened anteriorly by 

 projecting orbits; dorsal surface unarmed, or tuberculated, or with a 

 few long spines; margins \\dthout a series of elongated lateral spines, 

 but often with a strongly developed lateral epibranchial spine, pre- 

 ceded by some smaller spines. Spines of rostrum well developed. 

 Eyes retractile within roomy, projecting, tubular orbits. The anten- 

 nae have the basal joint considerably enlarged and armed distally 

 with one or two spines ; the mobile portion is sometimes concealed by 

 the rostrum, sometimes exposed. Merus of external maxillipeds 

 broader than ischium and notched at internal angle for insertion of 

 palp. 



The chelipeds in the male have the palms enlarged and the fingers 

 either arched and meeting only at tip or in contact throughout. 

 Ambulatory legs rather short. 



Occurs from Cape Fear, North Carolina, to Bahia, Brazil, and 

 from Cape St. Lucas, Lower California, Mexico, to Ecuador; also 

 sparingly in the Indo-Pacific region. Low tide to 163 fathoms. 



KEY TO THE AMERICAN SPECIES OF THE GENUS MACROCOELOMA ^ 



A^. Carapace with fewer than 7 spines on its posterior half, or if with 7 spines, 

 some of them are small. 

 B^ Basal antennal segment armed with only one spine or sharp tubercle. 

 C Rostral horns separated by an interspace which is narrow or pointed at 

 base. 



D". Postero-lateral projections narrow, spinelike trispinosum, p. 466. 



D^. Postero-lateral projections broad, bladelike. 



E'. Postero-lateral projections very broad, their margins continuous 



with marginal lines of carapace trispinosum nodipes, p. 468. 



E^. Postero-lateral projections less broad, their margins making an 

 angle with marginal lines of carapace, trispinosum, variety, p. 468. 

 C2. Rostral horns separated by an interspace which is broad and rounded at 

 base. 

 D'. Carapace deeply sculptured or areolated between the two postero- 

 lateral spines. Rostral spines short and stout. 

 E'. Postero-lateral spines directed obliquely backward. 



subparallelum, p. 480. 



" Arctopsis tesselata White (nomen nudum), List Cnist. Brit. Mus., 1847, p. 5, from the West Indies, 

 may be a Macrocoeloma. Type not in British Museum. 



