BRYOZOA. 313 



Nlcholsonella.] 



Genus NICHOLSONELLA, Ulrkh. 



Nieholsotiella, Ulrich, 1890, Geol. Sur. 111., vol. vili, 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. Zooecial 

 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 zooecia; minutely 

 granulose in fully matured examples. Walls of both sets of tubes thin, and in the 

 peripheral region traversed longitudinally by minute tubuli. With agaa perforated 

 calcareous deposit fills the interzooecial spaces in which the walls of the mesopores 

 become unrecognizable. Mesopores with thicker and generally more numerous 

 diaphragms than the zocecial tubes. In the axial region of transverse sections of 

 the erect forms the tubes are very unequal. 



.Type: N.ponderosa Ulrich, Geol. Sur. Ill, vol. viii, p. 422, 1890. 



This is a Lower Silurian genus with rather uncertain aflfinities. 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 Heterotrt/pa. The type is one of the earliest as well as 

 one of the most complicated and interesting of the Trepostomata, 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, 

 Illinois. 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,, -A^. 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. cumulata is described 

 in my Illinois work from the upper beds of the Hudson river group at Wilmington, 

 Illinois. 



*mterotrvva vaupeli Ulrloh, 1883, Jour. Oin. Soo. Nat. Hist., vol. vi. p. 85. 



