PALAEONTOLOGY OF IOWA. 667 



THIS rock, though developed over a large area in the State 

 of Iowa, has thus far proved comparatively unproductive of 

 fossils. It was originally designated by Dr. OWEN the "Brec- 

 ciated limestone"; and the rock, in this condition, contains 

 few fossils. Farther to the southward the brecciated character 

 is partially or entirely lost, and the rock becomes a regular- 

 bedded and important formation, as I have already shown 

 in the geological report. 



It has been necessary, therefore, in order to illustrate its 

 palaeozoic characters, to go beyond the limits of the State ; 

 and consequently the illustrations are restricted to a few 

 species, some of which occur within the State, while others 

 are from the nearest authentic localities of this rock upon 

 the Mississippi river. 



Litliostrotion mamillare. 



PLATE XXIV. FIG. 5 a, b. 

 Stylina : LESUEUU, 1832. 



Astrea mamillaris : CASTELNAV, Terrains Sil. do 1'Amerique du Nord, PI. 24, f. 5, 1843. 

 Axinura canadensis : IB. Id. PI. 24, f. 4. 



Acrocyathus floriforme : D'ORBIGNY, Prodrome de Paleontologie, T. i, p. 160, 1850. 

 Lithost ration basalliforme : OWEN, Report of Wisconsin, Iowa and Minnesota, Tab. iv, f. 5 



and 6, 1852. 



Lithostrotion mamillare : EDWARDS and HAIME, Monograph des Polypiers fossils, pa. 433, 



pi. 13, f. 1, 1 a and 1 b. 



CORAL massive, or more rarely in subdendriform tufts. 

 Calyces circular or polygonal, varying according to the de- 

 gree of proximity of the individuals, very unequal, profound : 

 the centre is monticuliform, and the summit terminated by 

 a salient columella, which is sometimes compressed and sub- 

 cristiform. A very small septal fosset on each side, in the 

 direction of the longer horizontal axis of the columella, a 

 little deeper on one side than on the other. Number of rays 

 variable, according to the size of the calyx, from 18 to 46 ; 

 the ray-chambers slightly concave in the upper part, a little 

 convex in the middle, and again concave below. 



