476 GEOLOGICAL SURVEY OF IOWA. 



and I had hoped, before the conclusion of this Report, to be 

 able to present some fossil remains from this group. The 

 examinations yet to be made will be more especially directed 

 to the region occupied by the Coal measures and the Gypsum 

 formation, and we may hope to present some more exten- 

 ded information regarding these formations in a succeeding 

 Report. 



FOSSILS OF THE HAMILTON GROUP. 



CORALS. 



ACERVULARIA ( SCHWEIGGER ). 



" CORAL compound, fasciculate, or more often massive, in- 

 creasing by calicular budding ; cells presenting two separated 

 walls as in Aulophyllum ; rays well developed between the 

 walls, but much less in the central area ; no columella ; 

 transverse septa little developed". EDWARDS and HAIME, 

 Mon. Polyp. Fos. p. 414. 



Acervularia davidsoni ? 



PLATE I. FIG. 8 a, b. 



Acervularia davidsoni : EDWARDS and HAIME, Monograph des Polypiers fossils, etc. 

 pa. 418, pi. 9, f. 4, 4 a & 4 b. 



CORAL astrgeiform ; surface plane or subhemispherical ; 

 cells irregularly polygonal, unequal, the walls slightly zig- 

 zag ; interior wall scarcely defined, and often marked only 

 by the abrupt downward bending of the rays, which form 

 within this area a deep cup ; rays 32 to 40 in the outer di- 

 vision of the full-grown cells, often 20 in the younger cells, 

 distinctly denticulate towards the sides, less distinctly to- 

 wards the centre, in which there is sometimes a papilliform 



