Habitat and Occurrence. 



87 



occasional presence in examples from these and the similar sandstones of the 

 Waverly group, of a few root spicules which are in the condition of hydrated 

 oxide of iron (limonite), as well as from the iron stains whicih frequently 

 accompany much of the material from all such rocks. The porous sandy 

 matrix of both horizons has been an efficient aid to the removal of the 

 skeleton. Where replacement has gone on in the presence of abundant 

 decomposing organic matter, as in the bituminous rocks of the Quebec 

 group, Utica slates and Marcellus shales, the deposition of pyrite is 

 complete and sometimes excessive ; so that the form of the spicules may 

 become obscured by pyritous enlargement. 



In the calcareous shales of the Keokuk group in Indiana, the pyritous 

 i-eplacement is often retained. The fact that it is not always present but that 

 in the majority of instances it has been lost, leads to the inference that the 

 removal of the pyritized skeleton is constantly in process. It is, however, to 

 the sponges of this horizon that is due such knowledge as Ave now possess of 

 the ultimate spicular structure in the DicTTOspoNGiDiE. 



Although the sandstones of the Devonian and lower Carboniferous have 

 proven unsatisfactory media for the retention of the skeleton they have well 

 preserved the form of the sponges. They have been, at times, subjected to 

 slight distortion from compression, but this disfiguration is always much less 

 in these rocks than in those more highly laminated and of finer texture. The 

 impressions of the interior and exterior of the skeleton are often so sharply 

 defined as to afford a very clear conception of the structure of the network 

 and the ornamentation of the surface, the slight discoloration of superficial 

 ridges and laminae by iron stains frequently aiding such conceptions. While 

 specific and generic characters in this family are so largely dependent upon 

 variations of form, this mode of preservation is by no means unsatisfactory, 

 as the shape of the sponge is usually better preserved than in more 

 schistose sediments. 



Habitat and Mode of Occijrbence. 



The prevalence of these Dictyosponges in the sandy sediments of the 

 Chemung group is in strong contrast to the distribution of existing hexactin- 

 ellids over the present sea-bottom. The Chemung formation of the central- 

 western part of the State of New York is essentially a succession of arenaceous 

 beds in the condition of compact sandstones and sandy shales, with occasional 

 layers of argillaceous shale and, toward the upper part of the series, with 

 gravelly conglomerates. In all of these sponge remains have been found, but 

 the occurrence of isolated specimens alone can not be relied upon to determine 



