42 GEOLOGICAL AND NATURAL HISTORY SURVEY OF CANADA. 



Pachydictya HEXAGONALIS. (N. Sp.) 

 Plate IX., figs. 2-2^. 



Zoarium, a bifoliate expansion of unknown dimensions. The frag- 

 ment illustrated is two mm. thick. Surface nearly even. Apertures of 

 the zooecia mostly hexagonal, often quadrate or polygonal ; arranged 

 in rather irregular diagonally intersecting series, thirteen in five mm. 

 At intervals of five mm. clusters of cells of slightly larger size than the 

 average are formed. These are made more conspicuous by the thicken- 

 ing of the interspaces. The cells forming these clusters vary consider- 

 ably in size, the largest being one-third larger than those of the 

 ordinary size, while others are smaller than they. The interspaces are 

 rounded and slope down into the apertures of the zooecia, there being 

 no sign of a peristome. 



A vertical section shows that near the median lamina) the zowcia are 

 separated by exceedingly thin walls, that the vesicles are first devel- 

 oped after the tubes begin to bend outward (l e. above the primary 

 cell), that the vesicles thus cut impart an obscurely beaded character 

 to the middle third of the height of an interstice, and that in the outer 

 regions the vesicles are filled with solid tissue, in which exceedingly 

 minute dots are closely ranged into vertical series, causing the tissue 

 to appear vertically lined. The zooecial tubes are crossed by from 

 three to six unequally distributed diaphragms. The whole section has 

 an unusually irregular aspect. 



From a tangential section we learn that the primary cell, or that 

 portion of the zooecial tube which rests upon the median lamina, has 

 very thin walls, is of oblong-quadrate shape, that its outline soon 

 changes to lozenge-shape and then becomes elongate sub-hexagonal, 

 with the anterior half rounded and the posterior half with the two 

 sides and end slightly concave. Soon the posterior end is cut away 

 and a vesicle formed, the opening now being of sub-circular form. 

 After this the interspaces become thicker gradually, and at lawt solid 

 with numerous minute dark spots. This T consider to have been the 

 normal development of the zoarium, and all the stages described may 

 be traced in a single judiciously prepared section. Such a section must 

 cut the zoarium a little obliquely, so as to show its structure at various 

 depths from the surface. 



The development of the zooecia, as above described, is very much as 

 in certain species of Ptilodictya (e. g. P. magw'fica, Miller and Byer), 

 but in that genus the interstitial spaces are always solid, and vertical, 

 sections could in no case be confounded with similar sections of Pachy- 

 dictya. 



