BRYOZOA. 537 



FEXESTELLA RTTDIS Ulrich. 



PI. XLIX, flg. 3-3d. 



Zoarium flabellate. usually forming but little undulated sub- 

 circular expansions, attaining a diameter of 7 cms. or more. 

 Numerous root-like strong and smooth processes strengthen the 

 small base from which the branches diverge. These are strong, 

 moderately rigid, and bifurcate at intervals varying from 5 to 

 15 mm. On the obverse the branches are angular, the keel 

 strong but not very prominent, and carries large elongated 

 nodes about two to each fenestrule. The elevations of the nodes 

 varies in different examples being very prominent in some and 

 scarcely perceptible in others. Age and state of preservation 

 largely account for the variation. The average width of the 

 branches also varies from 0.28 to 0.33 mm. In all the average 

 strength is somewhat greater near the base than at the outer 

 margins: their number in a given space is, however, fairly con- 

 stant, there being usually sixteen or seventeen in 1 cm. Fenes- 

 t rules elongate elliptical, twice as long as wide, with ten and 

 one-half in 1 cm. Dissepiments depressed, comparatively weak, 

 rarely half as wide as the branches. Zooecia in two alternating 

 ranges, eighteen in 5 mm., three to each fenestrule, with rather 

 large circular apertures, and slight peristome, separated by in- 

 terspaces of nearly the same width as their diameter, which is 

 about 0.12 mm. When well preserved the interspaces and dis- 

 sepiments are faintly striated. 



Certain shallow cavities on the sides of the keel (see fig. 3b,) 

 may represent ooecia. 



On the reverse the branches are strongly rounded, often irregu- 

 larly tuberculose, at other times only thickened at their junc- 

 tion with the dissepiments. Between the nodes the surface is 

 smooth or faintly striated. 



This is one of the most persistent and characteristic species 

 of the Keokuk group, and is easily distinguished from its asso- 

 ciates by the robust and rather coarse aspect of its fronds. F. 

 burlingtonensis is nearly allied, but has larger fenestrules. The 

 common European F. plebeia McCoy is also closely related but 

 differs in a like manner. 



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