186 IOWA STUDIES IN NATURAL HISTORY 
inner wall of the branchial chamber. The under surfaces or 
edges of the lamelle of this gill are brushed by this third blade, 
which is, I suppose, attached to the first maxilla, although the 
condition of the specimen was such that this point could not 
be satisfactorily determined. 
This method of applying water to the gills is evidently the 
same as described by me as found in Gecarcinus; but in that 
ease the brushes were not so flat, more like bottle-brushes in 
fact, and the gills themselves more completely filled the bran- 
chial chamber. 
It would seem that the respiratory arrangement in Ucides is 
still more highly specialized than in Gecarcinus in that a def- 
inite portion is set apart and specialized for air-breathing, while 
aquatic respiration is effected much as in the last species. 
Ucides, it should be remembered, lives in the water when the 
mud-flats are submerged; and on the mud, where aerial respi- 
ration would be extremely useful, when the tide is out. Thus it 
has in effect a double set of respiratory organs analogous, but 
not of course homologous, to those of such an amphibious 
vertebrate as Necturus. This arrangement is in fact an inter- 
mediate stage between that of Gecarcinus, which spends a good 
deal of time on land, but depends on a modified gill respiration ; 
and that of Birgus latro which is almost wholly terrestrial and 
depends upon an entirely different respiratory apparatus which 
closely resembles lungs, having but a vestige of gills left in the 
bottom of the branchial chamber. 
Another land-crab secured on the mud-flats has the carapace 
light blue, almost sky-blue, and the legs purple. It was very 
active and aggressive. 
A much flattened form, allied to Sesarma, had orbicular 
chelz with short stubby fingers and a serrate row of spines on 
the anterior edge of the merus of each walking leg. A most pe- 
culiar little species, a Petrolisthes, was almost carmine and echar- 
acterized by much flattened chele, the anterior edge of which 
was almost like a knife-blade. Its most unusual feature, how- 
ever, was the antenna, which was very long and filiform and 
finely annulated throughout. This is the only brachyuran erab 
with antenne resembling those of the macrurans collected by the 
