1886.] NATURAL SCIENCES OF PHILADELPHIA. 47 



Nicholson and Murie. I have seen many specimens with a sur- 

 face covered by rounded monticules similar to those of Stromato- 

 pora monticulifera, Winchell, and the tops of these monticules 

 radiated by the convergence of the before-mentioned compressed 

 vermiform crests representing the pillars of an ordinary Stro- 

 matopora. I had photographic representations of such natural 

 surfaces on the plates pertaining to my old manuscript essay on 

 Stromatopora, besides figures of vertical sections which exhibit 

 the structure better than any description can do it. 



As a second aberrant form of Stromatoporoids the authors 

 pointed out a form to which they gave the name Pachystrovia. 



The tj^pe form occurs in the Niagara group and is described as 

 consisting of subhemispherical masses formed of a concentric 

 succession of very thick laminae from one to two lines in diameter, 

 which are in direct contrast, not separated by inter-laminar 

 spaces except occasionally by narrow irregular intervals, conse- 

 quently there are also no radial pillars. Under the microscope 

 the great laminse are formed of an indistinct porous calcareous 

 tissue principally composed of irregular vertical fibres placed at 

 some distance from one another, and only clearly brought to view 

 by the use of polarized light. Numerous delicate but irregular, 

 generally remote vertical vermicular tubules without distinct 

 walls are said to perforate the mass, but such poi'es could not be 

 discovered on fractured surfaces by the use of a simple lens. 

 The surface of weathered specimens also shows radiating branched 

 subdermal canals placed round numerous independent centres, 

 precisely as in the so-called Ccenostroma. 



Several figures of sections through Pachystroma are given, 

 which, together with the above description, convince me that the 

 authors had under observation imperfectly preserved specimens 

 of Stromatopora, which occur abundantlj'^ in the Niagara group ; 

 those found in the vicinity of Lockport in calcified condition, 

 particularly correspond with the given descriptions. They have 

 generally lost the finer structure, but exhibit distinctly a compo- 

 sition of concentrically superimposed bands consisting of an 

 almost structureless, compact, calcareous mass, cut into thin 

 slices, showing under the microscope the above described fibrous 

 texture. 



On examination of a good many specimens exceptionally a spot 

 may be found in which the structure is better preserved, and in 



