BRYOZOA. 209 



Phylloporina.l 



and Phyllopora, King, two genera that previous to 1888 were commonly employed in 

 designating species of Phylloporina. The three genera agree in having similarly 

 anastomosing zoaria, but the first belongs unquestionably to the Chilostomata, and is 

 not known in rocks earlier than the Cretaceous. In the second the zoaria are short 

 and constructed in every respect like those which characterize the Fencstellidce. 

 Phyllopora was not derived from the Lower Silurian Phylloporina, but from some 

 type of Polypora subsequent to the extinction of the Silurian forms. Yet I am much 

 inclined to believe that all three genera were derived successively from the same 

 primal stock not from each other and that we have here merely a case illustrating 

 the "tendency to variation in certain directions." 



Compared with Drymotrypa, Ulrich, a genus beginning in the Trenton, and con- 

 tinuing to the Lower Helderberg, the present genus is distinguished chiefly, perhaps 

 solely, by the anastomosis of the branches, these being dichotomously branched and 

 free in Drymotrypa. In the Carboniferous genus Chainodidyon, Foerste, the zooacia 

 are somewhat shorter, and the back or '' reverse " flatter and marked concentrically 

 instead of longitudinally. 



PHYLLOPORINA SUBLAXA Ulrich. 



PLATE IV, FIGS. 1-7. 



Phylloporina sublaxa ULUICU, 1890. Jour. Cin. Soc. Nat. Hist., vol. xii, p. 179. 



Zoarium an undulating flabelliform expansion, attaining a diameter of 5 cm. or 

 more, consisting of irregularly inosculating slender subcylindrical branches, varying 

 in width from 0.3 to 0.6 mm., but averaging about 0.45 mm. Fenestrules large, sub- 

 acutely elliptical, varying considerably in shape and size, generally two or three 

 times longer than wide ; measuring, longitudinally, the average number in 1 cm. is 

 between five and six ; transversely, nine or ten in the same space. These measure- 

 ments apply to the Tennessee specimens. In the Minnesota form of this species the 

 fenestrules are smaller, averaging between six and seven in 1 cm. lengthwise. 



Reverse of the Tennessee specimens strongly rounded, nearly smooth, or with faint 

 longitudinal striae. In very young examples the latter would probably be more 

 distinct. Figure 2 on plate IV represents an enlargement of the reverse of a small 

 fragment obtained from the lower part of the limestone at Minneapolis, Minnesota, 

 by splitting a block of limestone. As usual under such conditions the outer layer of 

 sclerenchyma has adhered to the opposite side of the matrix and exposed a more 

 youthful stage in the development ot the zoarium, in which the reverse side was 

 strongly striated. 



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