flexed. downward, and has usually a. sharp prominent tooth either side - x. 
exterior.to the inner antennz. 
The genus Megalopa, Leach, as now. accepted, embraces two distinct ns üf 
species—the JM. Montagut aud armata for which it was instituted by Leach, 
and the 7M. mutica of Desmarest. The former (the true Megalopz) have the. 
beak nearly horizontal, with rarely a tooth either side, and there isa reflexed 
spine on the ventral surface of the first joint of the 8 posterior legs. The latter 
has the beak bent downward vertically, and either side of it there i 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 sete rather longer than the tarsus, while the descriptions of the 
suutica make mention of no such setze. 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 ZM. mutzez 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 Galathaaa. 1I had examined several species before 
I discovered this habit with regard to the posteriorlegs. 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 off 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 setz at the extremity of the posterior tarsi. In one division, 
including the AM. inermi s,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 legzs 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 sete: 
somewhat shorter than the tarsus. 
There is still another group in which the front is horizontal and tricuspidate,. 
the inner antennze when retracted being exposed in the interval between the beak 
or inner cusp and either outer, lving in view as in Plagusia. 
With these explanations we give the characters of the genera. 
1. MouorzPis, Szy.—Carapax fronte tricuspidatus sed valde deflexus ideoque 
frons superne visus medio non acutus sed truncatus. Pedes 5ti minores, super 
carapacem szpe restantes, depressione ad eos recipiendos abruptá, 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.—JMonolepis inermis, Say, typus est. 
2. ManzsriA, Dana.—Carapax fronte uti in ZMonolepi. Pedes 8 postici ad 
basin infra non armati ; 5ti minores, super carapacem sape restantes, depressione 
ad eos recipiendos parce concavà ; tarsis styliformibus, unguiculatis, spinis infra 
*Journ. Acad. Nat. Sci., Philad., i. 155. 
