288 THE PALEONTOLOGY OF MINNESOTA. 



LBatostoma. 



Genus BATOSTOMA, Ulrich. 



Batostoma Ulbich, 1882, Jour. Cin. Soc. Nat. Hist., vol. v, p. 154 ; 1890, Geol. Sur. 111., vol. vili, pp. 



379, 459 ; Foord, 1883, Contri. Micro-Pal. Cam.-Sil. rocks Can., p. 17. 



Zoaria irregularly ramose, with a large basal expansion. Zocecial walls thin, 

 and irregularly flexuous in the axial region, more or less thickened in the peripheral. 

 In the most typical species the walls are irregularly ovate, thick and ring-like in 

 tangential sections, with neighboring zocecia in contact only at limited points, the 

 mesopores numerous, closed at the surface, and irregular in shape and size, and the 

 acanthopores abundant and with a larger central cavity than usual. Species vary 

 from these to forms having polygonal, thin-walled zooecia and very few mesopores 

 and acanthopores. Diaphragms strong, horizontal, complete, few or wanting in the 

 axial, more or less abundant in the peripheral region. In the axial region of trans- 

 verse sections the tubes are divisible into two sets, one larger than the other. 



Type : B. impUcatum Nicholson, sp. 



Beginning in the Birdseye limestone and shales of Minnesota with B. fertile, 

 B. magnopora, and an undescribed species, it is evident that, the same as with 

 Callopora and Dekayella, the primitive stock of the genus was of the simplest type. 

 Mesopores were few, the zocBcia angular and thin-walled, and acanthopores both 

 few and very small. And yet the first of these species varies by most gentle degrees 

 to forms (5. fertile var. circulare) having numerous mesopores, circular zocecial aper- 

 tures, and thick walls — in short to a form that approximates the most typical and 

 fully developed species of the genus. 



In B. implicata and B. jamesi Nich. sp., of the lower half of the Cincinnati group, 

 the zooecia are characterized by thick ring-like walls, irregularly oval apertures, 

 numerous mesopores and acanthopores. Although closely allied, B. variabile differs 

 conspicuously from them in having few mesopores and polygonal zooecia. Being a 

 later species, and one that doubtlessly is a direct descendant of those forms, we are 

 justified in assuming that a reversion in structure toward the primitive type took 

 place during the closing iimes of the Lower Silurian. While such a tendency seems 

 to have been general it was by no means universal, since at certain localities (Stony 

 Mountain, Manitoba, Iron Ridge and Delafield, Wisconsin) where the conditions for 

 the development of these Bryozoa seems still to have been eminently favorable, the 

 genus is represented by species with abundant mesopores. 



A similar reversion had already taken place during the deposition of the Utica, 

 in which two of the species, both as yet undescribed, are so simple in their structure 

 that their really intimate relations to B. jamesi might not be suspected. It is only 



