48 PROCEEDINGS OP THE ACADEMY OP [1886. 



those I can see that each one of the described large, thick laminae 

 consists of a number of delicate subordinate laminae, separated as 

 usual by minute interlaminar spaces, intersected with pillars. 

 In my opinion, therefore, the so-called Pachystroma merely repre- 

 sents a certain imperfect state of preservation of several of the 

 ordinary species of Stromatopora found in the Niagara group. 

 The grouping of the smaller structural elements into broader 

 bands is almost universally the rule in Stromatopora. Any ver- 

 tical section almost, in which the delicate laminae and interstitial 

 spaces alternate through the whole thickness of the mass with 

 great uniformity, will, if we direct our attention to it, show a 

 grouping of a number of such laminae into one broader, occa- 

 sionally narrower bands, discernible by a darker shade of color 

 on the junction line of two of such bands. It indicates tempo- 

 rary short interruptions in the regularity of the progressing 

 growth, as I have intimated previously, while giving a description 

 of the structure of Stromatocerium. 



Prof. Nicholson considers also Stromatopora densa of the Hel- 

 derberg group as a representative of his Pachystroma, but of this 

 form I can most positively assert that its structure does not differ 

 in any respect from the typical forms of Stromatopora. 



The specimens before me allow the recognition of the regular 

 superposition of laminae, supported by galleries of short, stout 

 pillars with greatest distinctness ; the orifices of the horizontal 

 channel expansions are in vertical sections, rather large, and the 

 ordinary interlaminar cell-spaces unusually small, which causes 

 the compactness and apparent density of the mass, as compared 

 with the open porous appearance of sections of other associated 

 forms of Stromatopora, the commonest of which is Stromato- 

 pora ponderosa, Nicholson. This latter form I had years ago dis- 

 tributed among my scientific friends under the name of Stroma- 

 topora tentilis, on account of its most elegant reticulated 

 appearance in vertical sections. In this respect it exceeds in 

 distinctness any other form of Stromatopora known to me. 



A third group of Stromatoporoids, aberrant from the ordinary 

 type, Mr. Nicholson named Clath.rodictyon. It is distinguished 

 from the type form by the flexuose course of the lamellae com- 

 posing the laminated section. They are continually turned up 

 and downward, the upturned portions representing the vertical 

 pillars of ordinary forms of Sti^omatopora, The interlaminar 



