BRYOZOA. 173 



Phaonopora.] 



As shown in the accompanying cuts, the internal structure brought out in 

 tangential sections is often almost exactly as in Ph. incipiens (plate XIII, fig. 17); 

 other sections, however, in portions at least, show a structure more in accordance 

 with fig. 7, plate XIII. 



It is evident that E. ( ?) limitaris and E. confluens are varieties of one species, 

 but because of their intermediate position between Escharopora and Phcenopom, it 

 is scarcely advisable to decide now which of the two names shall take the rank of a 

 species, and which that of a variety. Such decisions should be deferred till we are 

 better acquainted with the developmental history of fossil Bryozoa. 



Formation and locality. Not uncommon in the lowest third of the Trenton shales, at Minneapolis, 

 Minnesota. Also in the middle third of the shales at the same locality, but less common and of more 

 robust growth. A single specimen was observed among a lot of Bryozoa marking the lower shales, 

 collected by Messrs. Schuchert and Scofleld, near Preston, Minnesota. 



Mut. Reg. No. 5930. 



Genus PH^NOPORA, Hall. 



Phcenopora, HALL, 1852, Pal. N. Y., vol. ii, p. 46; ULEIGH, 1882, Jour. Gin. Soc. Nat. Hist., vol. v. 



p. 152, and 1890, Geol. Sur. 111., vol. viii, p. 392; FOERSTE, 1887, Bull. 

 Sci. Lab. Denison University, vol. ii, p. 157. 



Zoaria bifoliate, simple or branching, the base pointed and articulating into a 

 small basal expansion, the same as in Ptilodidya and Escharopora. Zooacial arrange- 

 ment regular, in longitudinal, diagonally intersecting and transverse rows, with 

 either the longitudinal or the diagonal series predominant. Two mesopores behfhd 

 each zooecial aperture, one on each side, or one behind the other. Primitive cells 

 elongate, commonly oblique or lozenge-shaped, at other times with the ends rectan- 

 gular, always arranged between straight, longitudinal walls. Monticules, or mere 

 clusters of large cells and mesopores, present when zoaria are wide enough. 



Type : Phcenopora explanata Hall, 1852, Pal. N. Y., vol. ii, p. 46. 



From the above description it is obvious that the presence of the two mesopores 

 is the only character to be relied upon in distinguishing the genus from Escharopora 

 and Ptilodidya. The genus attains its highest development in the Clinton group, and 

 in most of the species from that horizon the primitive cell has a peculiar oblique 

 shape that is not seen in the Lower Silurian representatives of the genus, nor in any 

 species of Escharopora, but is not uncommon among true Upper Silurian Ptilodidya. 

 As might be expected, it is among the unbranched species that the greatest resem- 

 blance to Ptilodidya obtains. Indeed, such species as Ph. ensiformis Hall, and Ph. 

 lonsdalei (Ptilodidya lonsdalei Vine) are in every respect, save in this that they possess 

 the characteristic mesopores, precisely like narrow species of Ptilodidya. 



In the following classification I have arranged the species in sections the same 

 as under Escharopora. Except in one instance, I have not been able to obtain 



