506 PALAEONTOLOGY OF ILLINOIS. 



base is traversed by high ridges which frequently coalesce and 

 form large irregular cup-shaped cavities. The dimensions are 

 variable, and the width of the faces varies from 5 to 12 mm., 

 but one of them usually wider than the others. The inequality 

 of the faces is often much increased by pressure. Margins sharp, 

 non-poriferous, essentially parallel. Each face is ornamented by 

 a series of somewhat curved, elongated, slightly depressed ma- 

 culae (dimples) extending in a pinnate manner from near the 

 middle of each face to the non-poriferous margin, about three 

 mm. long and one mm. or a very little more wide. The sepa- 

 ration of the dimples varies in different examples, but is ap- 

 proximately uniform in each. The specimens figured represent 

 the typical and commonest form. Apertures circular, often a 

 little oblique, about 0.2 in diameter, with more or less elevated 

 peristome, arranged in rows nearly parallel with the dimples, 

 about six in three mm. Interspaces about one-half greater 

 than the diameter of the apertures. Internal structure as re- 

 quired by the genus. 



There are probably three species of Prismopora in the Keo- 

 kuk group, which, mainly on account of the indifferent preser- 

 vation of their fragmentary remains, are now confounded. One 

 of these (represented by fig. 45) has one of the faces very broad, 

 and the dimples widely separated and not extending to the 

 margin. The second has a smoother aspect than the others, 

 the dimples far apart, and the faces subequal. The third is the 

 most abundant, and being identical with authentic examples of 

 Fistulipora trifolia Rominger, forms the basis of the above de- 

 scription. 



Position and locality: Keokuk group. Abundant at Keokuk, 

 Iowa, LaGrange, Mo., and Warsaw, 111. 



PBTSMOPORA MINIMA Ulrich. 



PI. LXXVIII, fig. 1-lc. 



Zoarium consisting of small, infrequently branching, triangu- 

 lar stems, with the faces subequal, flat or a little concave, and 

 about 1.5 mm. wide. Margins sharp, nearly straight and es- 

 sentially parallel, with the non-poriferous border mostly nar- 

 row, but at irregular intervals much widened to form smooth 



