150 THE PALEONTOLOGY OF MINNESOTA 



[Pachydiotya. 



of these diaphragms represents the floor of distinct zocecia which have succeeded 

 each other by direct sequence, the formation eventually of the present "tube" being 

 the necessary result. 



In tangential sections, obtained by grinding down into one of the faces of the 

 zoarium, we notice characters as follows : Beginning with the base of the zocecia, 

 i. e., the mesial laminae," we find them represented by a darkened space, (usually a 

 meandering streak across the deepest part of the section) crossed by two sets of 

 parallel lines, one, colorless, representing the "median tubuli" that are inclosed 

 between the two halves of the basal plate, the other, of a dark tint, the longi- 

 tudinally directed side walls of the zocecia and intertstitial vesicles. Imme- 

 diately above this space the zocecia are slightly elongate, with the anterior end 

 widest and rounded, and the posterior end usually truncated. Behind this is a 

 darkened narrow space which, though really an interstitial vesicle, often appears to 

 be a part of the zocecium. The two together are somewhat bottle-shaped. Directly 

 following this stage the zocecia become shorter, broadly elliptical in shape, and sepa- 

 rated from each other by narrow interspaces in which the elongated interstitial 

 vesicles are more or less plainly visible. In the next stage the vesicles are more 

 and more obscured by a seemingly structureless deposit of sclerenchyma, while the 

 bounding wall of the zocecia becomes more ring-like. If the section is a good one 

 and the preservation favorable, this wall will be seen to consist of a closely arranged 

 row of minute tubes, apparently of the same nature as the minute tubuli between 

 the mesial laminae. In the last stage observed (seen in a section showing the struc- 

 ture just beneath the surface of an old example) the interspaces are traversed by 

 one or two intertwining lines of minute dark spots (median tubuli) and a ring of 

 sclerenchyma, of light color and laminated structure, deposited on the inner side of 

 the zocecial wall. The maculae, consisting of aggregations of interstitial vesicles, go 

 through the same changes as the ordinary interspaces. 



Good transverse sections dividing the zoarium vertically, but at right angles to 

 the direction of growth, show, among other features, the minute tubuli between the 

 mesial laminae in a very satisfactory manner. A significant fact is that one of these 

 tubuli seems always to be placed immediately beneath the walls of both the zocecia 

 and the intercalated vesicles. This is true, 1 believe, of all the Rhinidictyonida', and 

 is strong evidence in favor of my view that the two sets of minute tubuli, horizontal 

 and vertical, prevailing in this family of Bryozoa, communicated with each other. 



A very similar form occurs near the river level at Ottawa, Canada, but as it pre- 

 sents several internal peculiarities, especially in the form and arrangement of the 

 primitive portion of the zocecia, I will pass it by with this mere mention. 



