﻿H. M. Seely — Genus Strephochetus. 33 



fossil appears coated with black and from its covering takes its 

 specific name. 



, S. JRichmondensis S. A. Miller. 



A free globular or sub-globular calcareous sponge, varying 

 in size from two-eighths to seven-eighths of an inch in section, 

 consisting of numerous irregularly concentric laminae, separated 

 by interlaminar spaces, filled in most cases with exceedingly 

 small twining canals, or rarely with minute vertical tubes. These 

 laminae are more dense than the intervening spaces and in those 

 examples in which the tubular structure chiefly prevails they 

 seem the basis from which the tubes radiate. These tubes are 

 arrested by a laminar covering and this becomes the floor for a 

 new set of tubes. This structure of laminae and tubes may be 

 many times repeated. 



The twining canals have the ordinary diameter of the stre- 

 phochetal structure, about 2 0V0 of an inch, the radiating tubes 

 vary from T oV to 4?o of an inch ; j±-q of an inch however is 

 the usual diameter. The irregularly concentric laminae in cut 

 and weathered specimens give the form the appearance of a 

 Stromatocerium. 



This fossil occurs in the upper Hudson River rocks at Rich- 

 mond, Ind., Madison, Ind., and at Turners, Ky. 



The description of the form, & Brainerdi, is hardly com- 

 plete without further reference to its remarkable structure. 

 While externally it has the stromatoporoid appearance of the 

 other members of the genus, its internal structure may in part 

 vary from the typical form, by the presence of furrows and 

 larger canals, both cylindrical and compressed. One might 

 readily suspect on making a transverse section of some parts of 

 the specimen, that he had in hand a coral, or possibly a coral 

 enveloped by a sponge. But after cutting and carefully exam- 

 ining many sections, and especially as he saw the strephochetal 

 portion at points giving way to furrows, and the gradual intro- 

 duction of cylindrical canals of large size, and then these, 

 pressing upon each other where the interstitial material became 

 less, until a section was angular, he would reach the conclusion 

 that all these various structures belong really to the same fossil. 



S. atratus so nearly resembles & ocellatus externally, that it 

 may be easily mistaken for this latter fossil. Its minute struc- 

 ture however differs from it, and is so obscure, that it was a 

 question whether it could be placed here. Still its general 

 character so nearly resembles the type species, and its structure 

 so closely repeats that of some specimens of S. Brainerdi, that 

 it is safe to place it here. 



In the paper referred to at the beginning of this article, 



mention was made of the microscopic character of Stromatoce- 



Am. Jour. Sci.— Third Series, Vol. XXXIT, No. 187, July, 1886. 

 3 



