146 THE PALEONTOLOGY OF MINNESOTA. 



[Pachydictya. 



sented by the marginal rows of apertures which are commonly of larger size, with 

 wider interspaces, and less regularly arranged than those of the central rows. Zooec- 

 ial tubes rising rather abruptly from the mesial laminae, the primitive cells with thin 

 walls, longitudinally arranged, of elliptical, semicordate, or subquadrate form, in 

 most cases partially separated from neighboring cells by small interstitial vesicles. 

 Toward the surface their walls are thickened, often ring-like, subelliptical in cross- 

 section, usually completely isolated, the interspaces solid excepting that they are 

 transversed by one or more, straight or flexuous, series of minute tubuli. One or 

 more (the number depends upon age of example) complete diaphragms in each 

 zocecial tube. Apertures usually elliptical, rarely subangular, the "closures" with a 

 subcentral small opening. Interspaces grano-striate, concave and forming a peri- 

 stome about the zocecial apertures, or thrown up into longitudinal ridges. Median 

 tubuli between the halves of the double mesial plate. 



Type : P. robusta Ulrich. 



The distinguishing characters of section a, which includes the type of the genus, 

 are (1) the wide, palmate or foliar zoarium, and (2) the maculae and clusters of large 

 zoo3cia. The section might be still further subdivided according to whether the 

 longitudinal arrangement of the zocecial apertures predominates, or that in diagon- 

 ally intersecting series. The latter would include the species robusta, everetti, foliata, 

 magnipora and hexagonalis, all, save the last, described by me from the lower beds of 

 the Trenton formation ; while the former would embrace the species occidentalis 

 Ulrich (upper Trenton), fenestelliformis (Nicholson), firma, giganiea, and splendens, 

 Ulrich (upper beds of Hudson Rjver group), and species obesa and turgida, described 

 by Foerste from the Clinton rocks of Ohio. 



In section b, the zoarium is narrow, and its margins subparallel, while the longi- 

 tudinal arrangement of the zooecia is always the predominating one. It seems that 

 maculae, or merely an unusual width of the interspaces, must always accompany the 

 clusters of large cells, and as the room was insufficient in these narrow zoaria for 

 their proper development, or, it may have been that their presence would have 

 interfered too greatly with the regular growth of the branches, they (the large cells) 

 are instead arranged along the margins, where we may assume, the necessary condi- 

 tions to have been afforded by the non-poriferous border, which is constructed essen- 

 tially upon the same principle as the maculae.* 



The following species are to be arranged under Section b: acuta (Hall) fimbriata, 

 pumila, and triserialis, from the Trenton ; alcyone, arguta, and rustica, of Billings, from 



* It is a fact worth remembering that as soon as the width of the zoarium of one of the paleozoic bifoliate Bryozoa 

 exceeds 4 or 5 ram., a maculum or cluster of cells larger than the average is found a short distance beneath the axes of 

 bifurcation. A still greater increase and we have a row of maculae or monticules along the center of the surface, Several 

 Instances of this kind are illustrated on the plates accompanying this volume. (See plates VII and VIII.) 



