8 [January, 



flexed downward, and has usually a sharp prominent tooth either side of it, 

 exterior to the inner antennae. 



The genus Megalopa, Leach^ as now accepted, embraces two distinct sets of 

 species the M. Montagui and armata for which it was instituted by Leach, 

 and the M. mutica of Desmarest. The former (the true Megalopae) have the 

 beak nearly horizontal, with rarely a tooth either side, and there is a reflexed 

 spine on the ventral surface of the hrst joint of the 8 posterior legs. The latter 

 has the beak bent downward vertically, and either side of it there is a prominent 

 spine or tooth ; the ventral surface of the base of the legs is unarmed. The M. 

 mutica is very closely related to Monolepis spinitarsus of Say, the only differ- 

 ence being that the extremity of the posterior legs in this species of Monolepis 

 bear 3 or 4 setae rather longer than the tarsus, while the descriptions of the 

 mutica make mention of no such setae. The posterior legs in Monolepis fold up 

 and overlie the carapax : but these legs are otherwise like the preceding, though 

 somewhat smaller, and it is probable that this habit in the M. mutica has been 

 overlooked, as these animals almost always swim with the posterior legs ex- 

 tended like the others, when taken and kept in a jar for examination, and they 

 also have them extended when walking. These legs do not resemble at all the 

 posterior pair in Porcellana or Galathaea. I had examined several species before 

 I discovered this habit with regard to the posterior legs. The animal also throws 

 the fourth pair of legs forward along or over the borders of the carapax, so that 

 the extremity overlies the bases of the eyes and the tarsi hang down in front ; 

 and at the same time the two preceding pair are folded up and lie against the 

 sides of the carapax outside of the 4th pair, or the 3d pair may be thrown for- 

 ward like the 4th. A Sooloo species, and another common oflf Cape of Good 

 Hope, were observed swimming with the legs thus disposed. 



Say's genus Monolepis* also embraces two groups, alike in the deflexed front 

 and the longish setae at the extremity of the posterior tarsi. In one division, 

 including the M. inermisi the tarsi are flattened styliform, and unarmed, with 

 either lateral edge sparsely furnished with minute hairs ; the fossa of the sternum, 

 along which the abdomen lies when inflexed, has a prominent trenchant border ; 

 the depression on the carapax for the posterior legs is rather abrupt and some- 

 what neatly defined; the body is very convex and obese, with the sides high and 

 vertical, and much wider behind than before, being gradually narrowed forward. 



The other division has the tarsi unguiform, compressed, and spinous below, the 

 antepenult spine always longest; the fossa of the sternum with flaring borders^ 

 the depression of the carapax for the posterior legs shallow concave ; the body 

 more flattened above, with the sides more oblique. This division corresponds 

 to Monolepis spinitarsus. 



Besides the preceding, there is another group of Megalopidea, examined by the 

 author, resembling Megalopa of Leach, except that the tarsus of the posterior 

 legs is narrow lamellar instead of unguiculate, and edged with longish setae 

 somewhat shorter than the tarsus. 



There is still another group in which the front is horizontal and tricuspidate, 

 the inner antennae when retracted being exposed in the interval between the beak 

 or inner cusp and either outer, lying in view as in Plagusia. 



With these explanations we give the characters of the genera. 



1. Monolepis, Say. Carapax fronte tricuspidatus sed valde deflexus ideoque 

 frons superne visas medio non acutus sed truncatus. Pedes 5ti minores, super 

 carapacem saepe restantes, depressione ad eos recipiendos abrupta, tarsis inermi- 

 bus, depressis styliformibus, paris postici non depressis, apice 3 4 setis longius- 

 culis (tarso paulo longioribus) instructo. Sterni fossa abdominalis marginibus 

 bene prominens et subacuta. Monolepis inermis^ Say, typus est. 



2. Marestia, Dana. Carapax fronte uti in Monolepi. Pedes 8 postici ad 

 basin infra non armati; 5ti minores, super carapacem saepe restantes, depressione 

 ad eos recipiendos parce concava ; tarsis styliformibus, unguiculatis, spinis infra 



*Journ. Acad. Nat. Sci., Phiiad., i. 155. 



