222 THE PALEONTOLOGY OF MINNESOTA. 



' [Atactoporella. 



character is the development and continuous presence of cystiphragms in the axial 

 region of the zoarium. In this respect the. species agrees with M. arborea and M. 

 grandis, and differs from all species of Homotrypa in which these structures should be 

 developed in the peripheral region only. Even granting the importance of this 

 distinction it should be understood that the present arrangement of the species 

 is regarded as provisional. 



Compared with M. arborea, the species under consideration is distinguished by 

 having larger zocecia and thinner walls. The internal differences will be appreci- 

 ated at once in comparing the various figures of the two species given on plate XX. 

 Homotrypa callosa, illustrated on the same plate, has thicker walls, fewer diaphragms 

 and no cystiphragms in the axial region. 



Formation, mid locality.— Hare in tlie Galena shales, near Cannon Falls, Minnesota. 



Genus ATACTOPORELLA, Ulrich. 



Ataetopora (part.), Uleioh, 1879. Jour. Gin. Soc. Nat. Hist., vol. ii, p. 119. 



Atactoporella, Ulkich, 1883, Jour. Cin. Soc. Nat. Hist, vol. vi, p. 247; 1890, Geol. Surv. 111., vol. viii, 



p. 370. 

 Peronopora (part.), Nicholson, 1881. "The Genus Monticulipora," p. 215. 



Zoarium generally forming thin crusts over foreign bodies, rarely lobate or sub- 

 ramose. Surface commonly with monticules. Zooecia with very thin inflected walls, 

 their apertures irregularly petaloid ; internally with cystiphragms. Mesopores angu- 

 lar, numerous, often completely isolating the zocecia ; at first open and distinctly 

 tabulated, but, when fully matured, largely or entirely filled by a deposit of scleren- 

 chyma. Acanthopores very numerous, varying in size with the species, encroaching 

 more or less upon the zooecial cavity. 



Type : Atactoporella typicalis Ulrich. 



The affinities of this genus are with Monticulipora on the one side and PeronopoYa 

 on the other. From the first it is distinguished by the abundant development of 

 mesopores and the more numerous acanthopores. These are not only usually 

 smaller in Monticulipora, but they also do not inflect the zooecial walls. In Peron- 

 opora, as restricted by me, the zoarium is bifoliate, and the mesopores are not filled 

 up with age in the manner characteristic of Atactoporella. 



There may be in all about twelve species of this genus known to me, six of 

 which are described from the Cincinnati rocks in the papers cited above. The same 

 series of beds afford at least two new forms, and the Trenton of Kentucky and Ten- 

 nessee two more, while the Birdseye and Trenton of Minnesota add the following 

 species to the list , 



