4(>0 PALAEONTOLOGY OF ILLINOIS. 



A peculiar group of species comprises B. imperfectum Ulr., B. 

 irrasum Ulr., and B. ottawense Foord. These differ from the 

 other species in having incomplete diaphragms, and ought, per- 

 haps, to constitute a distinct genus. 



BATOSTOMA VARIABILE Ulrich. 



PI. XXXV, fig. 4-4e, 5, and PL XXXVI, fig. 1. 



Zoarium exceedingly variable; encrusting, lobate, digitate, 

 ramose, and subfrondescent. Surface smooth, showing inappre- 

 ciably elevated clusters of larger cells. Walls of zooecia in axial 

 region thin, faintly and irregularly flexuous; much thickened in 

 the mature region; the tubes are polygonal, approach the surface 

 with a gradual curve, and are mainly in contact, with the divisional 

 line between those adjoining sharply marked. Apertures angu- 

 lar, averaging six in two mm. Mesopores angular, varying in 

 number, generally few as in the sections figured. Diaphragms 

 few in the immature region, three or four in the narrow cortical 

 region, the one or two nearest the surface concave; in the 

 mesopores they are moderately abundant. Acanthopores fairly 

 numerous, usually situated at the angles between the zocecia. 



The name "variabile" is given to this species because different 

 examples exhibit considerable variations in the mode of growth, 

 in the number and arrangement of the mesopores, in the 

 amount of thickening of the walls in the cortical region, and 

 some other features. 



This form bears much resemblance to the B. jainesi Nich., 

 which, however, has oval zooecia separated by many mesopores, 

 while this species has the zooecia polygonal and often in contact, 

 with fewer mesopores and diaphragms. 



Position and locality: A very abundant and characteristic 

 species of the upper beds of the Cincinnati group. It is found 

 at numerous localities in Ohio and Indiana, and at Savannah, 

 111., and Delafield, Wis. 



BATOSTOMA IMPERFECTUM Ulrich. 



PI. XXXV. fig. 3-3d. 



Zoarium ramose, branches subcylindrical, very robust, twenty 

 mm. or even more in thickness, dividing at short, irregular in- 



