254 THE PALEONTOLOGY OF MINNESOTA. 



[Aspidopora. 



0.28 mm. Zooecial walls very thin, in contact at limited points, yet leaving compar- 

 atively large and easily distinguished interspaces, which, because the prevailing 

 arrangement of the apertures is quincuncial, are commonly bounded by four concave 

 sides. In most cases each of these spaces is occupied by a single mesopore, averaging 

 0.1 mm. in width and a little more in length. Mesopores but little if at all more 

 numerous in the clusters than elsewhere. They may be however a little larger and - 

 less regular in shape. Acanthopores apparently wanting. 



This species may be nearly related to P. simulatrix Ulrich, but it is not its 

 young. This is shown best by thin sections, but the smaller size and more oval 

 shape of the zooecial apertures, the absence of maculae, and the larger size and dif- 

 ferent shape of the interspaces, alone are sufficient in discriminating between them. 

 Comparing internal characters these differences are emphasized, while another point 

 is added in the greater separation of the diaphragms in the mesopores. Tangential 

 sections are a little like those of P. insularis,'Ya,Y. filmorensis, but the zocecia in that 

 species are completely isolated, and the tabulation of both sets of tubes much more 

 crowded. It seems to me that Aspidopora parasitica Ulrich, is more closely related, 

 but differs sufficiently in its parasitic growth, more closely tabulated mesopores, and 

 in possessing small acanthopores. 



Formation and locality. — Rare in the upper third of the Trenton shales at St. Paul, Minnesota. 



Genus ASPIDOPORA, Ulrich. 



Aspidopora, Ulrich, 1882. Jour. Cin. Soc. Nat. Hist., vol. v, p. 155; 1890, Geol. Sur. 111., vol. viii, p. .373. 



Zoarium consisting of one, or two or more superimposed, thin expansions, each 

 1 mm. or less thick, rarely parasitic, generally free, with an epithecal covering on 

 the concave lower side ; typically coniposed, according to age, of frond one to many 

 subequal parts, each gently convex, with the zooecia increasing in size from their 

 margins to near their centers. Mesopores numerous, largest and best seen exter- 

 nally in the younger stages. Acanthopores usually present, always small. Dia- 

 phragms horizontal and closely set in the mesopores, usually wanting in the zooecial 

 tubes, but one or more cystiphragms occur in most of the latter. 



Type : A. areolata Ulrich, IJtica horizon, Cincinnati group. 



This genus is so closely related to Prasopora, Nich. and Ethr., jr., that for some 

 time I have considered the propriety of dropping the name in favor of theirs. But, 

 as I could not arrive at a wholly satisfactory conclusion, it seemed best to retain the 

 genus till we can learn more of the developmental history of this section of the 

 Monticuliporidce. 



