176 THE PALEONTOLOGY OF MINNESOTA. 
[Arthropora. 
The walls consist, first, of the original, transversely lined (? minutely perforated) 
investment, and, second, of an inner laminated deposit (see fig. 25). This structure 
prevails in all the typical genera of the Ptilodictyonide, but, unfortunately is rarely 
preserved. 
Every important feature noticed in vertical sections is represented in fig. 26. 
When plate XIII was lithographed the specimen thereon illustrated was the only 
one then available. When, several months later, the remainder of my collections 
from Wilmington, Illinois, was unpacked, I was fortunate enough to find seven more 
examples, three of them with the pointed basal extremity. 
The presence of mesopores distinguishes this species from Lower Silurian 
Escharopora, like E. maculata Ulrich, while their longitudinal arrangement serves to 
separate it from the unbranched Upper Silurian species of Phenopora. 
Formation and loculity.—Upper beds of the Hudson River group, at Wilmington, Illinois. 
Genus ARTHROPORA, Ulrich. 
Ptilodictya and Stictopora ( part.), of several authors. 
Arthropora, ULRICH, 1882, Jour. Cin. Soc. Nat. Hist., vol. v, p. 152; S. A. MILLER, 1889, North Amer. 
Geol. and Pal., p. 293; Unricn, 1890, Geol. Sury. Ill., vol. viii, p. 393. 
Zoaria bushy, spreading in a plane, composed of numerous, essentially equal 
segments; joints simple, bifurcating, or with several short lateral branchlets, the 
extremities solid and rounded for articulation with succeeding segments. Zocecial 
apertures elliptical, surrounded by a delicate peristome. Interspaces with one or 
more thread-like ridges, variously disposed, sometimes short and vermicular, at other 
times forming continuous longitudinal wavy lines, or ranged in a concentric manner 
about the apertures. Peristomes and ridges each with a row of minute papille. 
Interior with the primitive cell elongate, narrow, one or both hemisepta, and lined 
with minute dots (? median tubuli) between the zocecia in the peripheral region. 
Mesial lamine zigzag in transverse sections, without “median tubuli.” 
Type: Arthropora shafferi. (Stictopora shafferi Meek). Range, from base of 
Trenton formation to top of Hudson River group. 
This genus is closely related to Graptodictya, the only difference being that in 
the species of that genus the zoarium is continuous above the basal articulation, 
while in Arthropora it is divided into subequal joints. In certain of the internal 
characters, (e.g. the rows of interstitial dots) we are reminded of the Rhinidictyonide, 
but the general agreement with the Ptilodictyonide, especially in the absence of 
minute tubuli between the mesial lamin, precludes all likelihood of near relation- 
ship with Rhinidictya. 
