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 Ptilodictyonidce, 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 

 Es<;iiaropora, like E. maculata Ulrich, while their longitudinal arrangement serves to 

 separate it from the unbranched Upper Silurian species of Phcenopora. 



Formation and ZocaMtj/.— Upper beds of the Hudson River group, at Wilmington, Illinois. 



Genus ARTHROPORA, Ulrich. 



Ptilodictya and Stictopora ( part.), of several authors. 



Arthropora, Uleich, 1882, Jour. Cin. Soc. Nat. Hist., vol. v, p. 152 ; S. A. Miller, 1889, North Amer. 



Geol. and Pal., p. 293; Ulhich, 1890, Geol. Surv. 111., 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. Zooecial 

 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 papillae. 

 Interior with the primitive cell elongate, narrow, one or both hemisepta, and lined 

 with minute dots (? median tubuli) between the zooecia in the peripheral region. 

 Mesial laminae zigzag in transverse sections, without " median tubuli." 



Type : Arthropora shafferi {Stictopora shaferi Meek). Range, from base of 

 Trenton formation to top of Hudson River group. 



This genus is closely related to Graptodidya, the only difference being thali 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 Bhinidictyonidce, 

 but the general agreement with the Ptilodictyonidce, especially in the absence of 

 minute tubuli between the mesial laminae, precludes all likelihood of near relation- 

 ship with Bhinidictya. 



