BEYOZOA. 149 



Pachydietya.l 



Section a: Species in which the zoarium is not limited, and maculce or clusters of 

 large zocecia are present. 



PACHYDICTYA FOLIATA Ulrich. 



PLATE IX. FIGS. 1-5; PLATE X, FIGS. 5-10. 



Pachydictya foliata ULRICH, 1886. Fourteenth Ann. Rep. Geol. Nat. Hist. Surv. Minn., p. 73. 



Zoarium growing from an attached basal expansion into erect, thin fronds, 

 undulating and simple, or dividing palmately or irregularly; both sides cellulife- 

 rous ; attaining a hight and width of 50 mm. or more, but specimens larger than 25 

 mm. square are rare ; usual thickness about 1.2 mm., but in some old examples it is 

 quite 3.0 mm. Margin of fronds acute or rounded, often with a distinct non-periferous 

 border. At intervals of 3.5 or 4.0 mm. the surface presents solid, substellate spots or 

 maculae, that in most cases are on a level with the general plane of the surface, in 

 others occupying the summits of low monticules, while in rare instances they may 

 be even slightly depressed. These maculae usually appear smooth, but when well 

 preserved are seen to be finely grano-striate. Zooecial apertures large, oval, arranged 

 in regular diagonally intersecting series, in which fourteen or fifteen of the average 

 size is the usual number in 5 mm. In the immediate vicinity of the maculae they 

 are larger, attaining a size of 0.4 mm. by 0.3 mm., the average size in the spaces 

 between the maculae being about 0.3 mm. by 0.2 mm. There is a slight difference 

 also in the size of the apertures of the old and young specimens, they being largest 

 in the latter. Interspaces usually of less width than the zooecial apertures, concave 

 and forming a distinct peristome around the aperture in the young examples ; becom- 

 ing flattened and even faintly convex, also minutely granulose with age. Interstitial 

 vesicles seen at the surface in the youngest specimens only. 



In vertical sections the zooecial tubes arise rather abruptly from the mesial lam- 

 inae, the course to the surface throughout being also unusually direct. The prostrate 

 or primitive cells may be in contact, with a thin divisional wall ; but this is not the 

 rule since the interstitial vesicules are developed at the same time. The character 

 of the latter is clearly preserved for a distance of about 0.5 mm. on each side of the 

 mesial laminae, but beyond this they are filled with solid material in which they are 

 but illy traceable. Occasionally it is possible to detect faint dark lines passing vertically 

 through this solid filling, indicating that communication was maintained with the 

 horizontal median tubuli. The zooecial tubes are bordered on each side by a double 

 line, and crossed, according to age, by from one to five complete diaphragms. These 

 occur approximately on the same level in all the tubes, and at intervals correspond- 

 ing more or less nearly with the diameter of the tube. If my view is correct, each 



