117 Boíiany and Zoology. 
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 extended 
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 Galathea. I had examined 
several species before I discovered this habit with regard to the poste- 
rior 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 third pair may be thrown 
forward 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 de- 
flexed front, and the longish set: at the extremity of the posterior 
tarsi.. In one division, including the M. inermis, tbe 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 somewhat 
neatly defined ; the body is very convex and obese, with the sides high 
and vertical, and much wider behind than before, being gradually nar- 
rowed 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, ex- 
amined by the author, resembling Megalopa of Leach, except that the 
tarsus of the posterior legs is narrow lamellar instead of unguiculate, 
and edged wiih longish setze somewhat shorter than the tarsus. 
"There is still another group in which the front is horizontal and tri- 
cuspidate, the inner antenne when retracted being exposed in the in- 
terval 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. 
1l. Moworz?is, Say.—Carapax fronte tricuspidatus sed valde deflexus 
ideoque frons superne visus medio non acutus sed truncatus. Pedes 
bti minores, supe carapacem s:epe restantes, depressione ad eos reci- 
piendos abruptá ; tarsis inermibus, depressis, styliformibus, paris postici 
non depressis, apice 8—4 setis longiusculis (tarso paulo longioribus) 
instructo. Sterni fossa abdominalis marginibus bene prominens et sub- 
acuta.— Monolepis inermis, Say, typus est. | 
2. ManzsriA, Dana.—Carapax fronte uti in Monolepi, Pedes 8 
postici ad basin infra non armati; 5ti minores, super carapacem sape 
* Journ. Acad. Nat. Sci, Philad. i, 155. "The author is indebted for the privilege 
of examining a specimen of Say's M. inermis, to Prof. Lewis R. Gibbes of Charles- 
ion, S. C. Another related species was obtained by the author in the East Indies. 
