BRYOZOA. 313 
Nicholsonella.] 
Genus NICHOLSONELLA, Ulrich. 
Nicholsonella, ULRICH, 1890, Geol. Sur. Ill., vol. viii, pp. 374 and 421. 
Zoaria consisting of irregularly intertwining flattened branches or fronds growing 
from an expanded base ; or of laminar, free or parasitic, expansions only. Zocecial 
tubes subcylindrical, with diaphragms only moderately numerous; apertures circular, 
“ enclosed by a slightly elevated papillose peristome. Interspaces wide, occupied by 
numerous angular mesopores more or less completely isolating the zocecia; minutely 
granulose in fully matured examples. Walls of both sets of tubes thin, and in the 
peripheral region traversed longitudinally by minute tubuli. With age a perforated 
calcareous deposit fills the interzocecial spaces in which the walls of the mesopores 
become unrecognizable. Mesopores with thicker and generally more numerous 
diaphragms than the zoccial tubes. In the axial region of transverse sections of 
the erect forms the tubes are very unequal. 
Type: N. ponderosa Ulrich, Geol. Sur. T1., vol. viii, p. 422, 1890. 
This is a Lower Silurian genus with rather uncertain affinities. So far as our 
knowledge goes the position of the genus in classification seems to be in a measure 
intermediate between Constellaria and Leioclema. In another direction we note 
considerable resemblance to Heterotrypa. The type is one of the earliest as well as 
one of the most complicated and interesting of the T'repostomata, and on the whole 
appears to occupy a rather isolated position with respect to contemporaneous types 
of structure. It is therefore unfortunate,that the preservation of the most typical 
species of the genus is almost invariably unfavorable for microscopic determination 
of their internal peculiarities. Indeed, it is a noteworthy fact, that Trenton speci- 
mens of Nicholsonella are but rarely as well preserved as are associated Bryozoa of 
other genera. 
N. ponderosa was described from the “Lower Blue” or sponge beds at Dixon, 
ulinois. A nearly related species occurs at Beloit, Wisconsin, and in the middle 
third of the Trenton shales of Minnesota. Associated with the latter is the laminar 
species, N. laminata, about to be described. N. pulchra is a fourth species from 
the “Pierce” limestone of Tennessee, N. vaupeli* is abundant in the quarries at 
Cincinnati, Ohio, as is an undescribed and closely related form occurring higher in 
the series at several localities in Ohio and Indiana. Finally, N. cwmulata is described 
in my Illinois work from the upper beds of the Hudson river group at Wilmington, 
Illinois. 
*Heterotrypa vaupeli Ulrich, 18838, Jour. Cin. Soc, Nat. Hist, vol. vi, p. 85. 
