324 REPORT UNITED STATES ENTOMOLOGICAL COMMISSION. 
Mesopleurum. (Fig. 2.) The flanks are obliquely inclined. The epi- 
sternum is divided into a supra-episternite and an infra-episternite; the 
latter is trapezoidal, a little longer than broad, with a broad projection 
extending round in front, resting upon the mesosternum. The supra- 
episternite is sub-diamoud-skaped, the lower edge triangular, fitting into 
the infra episternite. 
The epimerura is divided into two pieces; the infra-epimerite is nearly 
as broad as long; the sub-epimerite is long, oblique, irregular in form, 
with three large projections from the surface. 
Trochantine broad and short. Coxa small compared with the trocli- 
antine, being about one-third as large. 
Metapleurum. (Fig. 3.) Exactly repeats the structure of the meso-» 
pleurum, except that it is a little longer, as the hind wings are larger 
than the anterior pair. Coxa and trochantine the same as in the meso- 
thovq.x. 
Sternum. 
Prosternum ( Pteronarcys californica). (Fig. 4.) Represented only by 
a swollen fold in front of the insertions of the legs, and by a gill-bearing 
membranous swelling behind. In Acroneuria abnormis there is a broad, 
large, scutellate chitinous piece. 
Mesosternum ( Pt . californica). (Fig. 5.) This sternite consists of two 
portions, (1) a raised, rounded sclerite (prsesternite) longer than broad, 
and situated on the front of the sternal area, between the two anterior 
gills; (2) behind is the true sternum, which is a vexy broad, trans- 
versely-oblong sclerite, square on the sides, and about one-fourth as 
long as broad, and somewhat curvilinear. In Acroneuria abnormis the 
mesosternum is divided into (1) a large pnesternite, which is broad and 
triangular; and (2) a large trapezoidal sternite. 
Metasternum. (Fig. G.) The same as in the mesothorax, but slightly 
larger. Behind the sternite, on both meso- and metathorax, are in each 
segment two deep fossm, extending probably into the entothorax (medi- 
and postfurca). In Acroneuria the metasternum is the same in form as 
the mesosternum, but the pnesternite is shorter and broader. 
THE UROSOME (ABDOMEN). 
In Pteronarcys californica (Pl.XLIY, figs. 7-9) there are ten abdominal 
segments (uromeres). The tergites are ten in number, the first broad 
and well developed, the tenth small and very short, with a median tri- 
angular projection (supraanal plate); the segment is entire but very 
short eternally. There, are no pleurites, except nearly obsolete mem- 
branous folds on the first and second uromeres, on which the first and 
second pair of spiracles are situated ; on the other uromeres the remain- 
ing six pair are situated on the lower edge of the tergites. From the 
hinder edge of the eighth urosternite two short, stout spihes project 
