No. 391.] NORTH-AMERICAN INVERTEBRATES. 595 
KEY FOR THE DETERMINATION OF AMERICAN FRESH-WATER 
Bryozoa.! 
a, Anus opening inside the tentacular corona [Endoprocta]. Urnatella 
gracilis. 
a2. Anus opening outside the tentacular corona, which is capable of being 
retracted [Ectoprocta]. 
b;. Zocecia sharply separated from each other; no epistome [Gymno- 
lemata]. 
; Zoæœcia cylindrical, arising from stolons; aperture terminal. 
Pottsiella erecta. 
€. Zocecia club-shaped, no stolons ; aperture lateral. Pa/udicella 
ehrenbergii. 
4,. Zocecia confluent, epistome present [Phylactolæmata]. 
cĉ Statoblasts without thorns, rounded at ends 
d, 20-22 tentacles arranged nearly in a circle; statoblasts 
without peripheral float. /7edericella sultana. 
də. 38-60 tentacles arranged in form of horseshoe. 
elliptical statoblasts with a peripheral float oe 
é; Free statoblasts elongated ; proportions, 1: 1.53 t 
a Plumatella princeps. 
ez. Fras statoblasts nearly circular, 1: 1 to 1:1 
J Free ends of zocecia fairly distinct from basal 
tubes, cylindrical or irregularly constricted. P/u- 
matella polymorpha. 
J» Free ends of zocecia mere conical elevations of 
basal tubes, covered with white spots. P/umatella 
punctata. 
ĉa» Statoblasts acutely pointed at both ends, without thorns. 
Lophopus cristallinus. 
c} Statoblasts with anchor-shaped thorns. 
d, Zoéids form rosettes on a gelatinous base, often attaining 
great size. Pectinatella magnifica. 
da Stock caterpillar-like, with broad sole. Cristatella mag- 
nifica. 
Literature on Fresh-Water Bryozoa.— Allman, G. J., A Monograph of 
the Fresh-Water Bryozoa, London (Roy. Soc.), 1856. — Hyatt, A., Observa- 
tions on Polyzoa, Suborder Phylactolemata, Proc. Essex Institute, IV and 
V, 1866-1868. — Leidy, J., Urnatella Gracilis, Journ. Acad. Nat. Sci. Philad., 
1883. — Potts, E., On Paludicella Erecta, Proc. Acad. Nat. Sci. Philad., 
1 The American Naturalist will undertake to determine and return any s 
mens that cannot be placed in the keys, and solicits correction and criticism for 
future revision. 
