MINUTE STEUCTURE OF STROMATOPOEA AND ITS ALLIES. 213 



Ibis state of things amongst fossils clearly referable to the same 

 great systematic group, the following being the most important : — 

 (a) In a Stromatoporoid already referred to from the Trenton 

 Limestone of North America, we find various remarkable peculi- 

 arities. In the first place, whilst the general aspect of the fossil 

 is unmistakably Stromatoporoid, we find that it possesses only 

 the crowded concentric lamina3 which characterize the typical 

 Stromatoporoids. These horizontal laminae are of considerable 

 thickness, and are separated, as usual, by well-marked iuterlami- 

 nar spaces; but the latter are quite open, and there is a total 

 absence of the vertical or radial calcareous pillars which occur in 

 the normal Stromatoporoids. Not only are the radial pillars 

 absent, but the whole mass is perforated by iunumerable vertical 

 canals, which are destitute of walls, and which open directly into 

 the successive interlaminar spaces, which they penetrate, as well 

 as into irregular open spaces or lacunae in the general laminated 

 skeleton. As seen in vertical sections, this form differs from the 

 normal Stromatoporoids in the absence of the radial pillars and 

 in the presence of the numerous vertical and large-sized canals 

 which divide the horizontal laminae into so many short sections of 

 an oblong form ; whilst in horizontal slices we no longer see the 

 truncated ends of the radial pillars. On the contrary, horizontal 

 sections show us nothing but the irregularly and obliquely- cut 

 edges of the horizontal laminae, often arranged concentrically round 

 minor centres, and sometimes showing indications of a minute 

 canal-system traversing their substance. This form is tolerably 

 abundant in the Trenton Limestone, and we have obtained it from 

 several localities. It is clearly generically distinct from the 

 normal Stromatoporoids, and must receive a distinct name. It 

 is, however, very probably the form upon which Hall founded his 

 genus Stromatocerium, and we would propose to revive this name 

 for it. Hall's Htromatocerium rugosum was obtained from the 

 Birdseye Limestone of the State of New York, which is only a 

 subdivision of the Trenton ; and as we are acquainted with no 

 typical form of Stromatopora (indeed with none but this) as occur- 

 ring in this formation, it is highly probable that our specimens 

 are identical with this. Hall abandoned his generic title simply 

 from the general resemblance of his specimens to Stromatopora, in 

 the broad sense of the term ; and our specimens certainly cannot 

 be referred to Stromatopora in the restricted sense of the name. 

 Whether our form is specifically identical with Hall's Stromato- 



