1886.] NATURAL SCIENCES OP PHILADELPHIA. 51 



Clathrodictyon which is not described by Prof. Nicholson, pre- 

 sents the weathered surfaces of the silicitied specimens, covered 

 with very stout rounded conical papilli of almost uniform size, 

 about half a millimetre in diameter and twice that distance apart. 

 Apices of the not cross-fractured papilli imperforate. In a 

 vertical section four or five well-demarkated stout lamellae occupy 

 the space of two millimetres, which is approximately the same 

 as one line. The interlamellar pillars are formed by upward 

 flexions of the lamella, and therefore we see the base of each 

 one excavated by an inverted funnel-shaped sinuosity ; and as 

 the pillars of the successiv^e laminae correspond, the apices of the 

 pillars of the inferior laminae become invaginated into the basal 

 funnel-shaped excavation of the next ones above, which being 

 the case through the whole thickness of the laminated mass, 

 makes it appear as if so many vertical columns intersected it 

 transversel}'. This structure was observed b}^ Mr. Nicholson in 

 another species of Stromatopora, and induced him to propose for 

 it the generic name Stylodictyon. 



The space between these pseudo-columns is bridged over by a 

 gentl}' downward curved portion of the lamellae, leaving corres- 

 pondingly large elongated interlaminar spaces between them, which 

 often are subdivided by arched vesicular plates obliquel}' and 

 irregularly traversing them. The mode of growth of this form 

 is the same as that of the former species ; it occurs in thick undula- 

 ting expansions, covered by an epithecal crust on the under side. 



An associate of these two forms, which I had named Stroma- 

 topora explanata, has been described by Prof. Nicholson under 

 the name of Stromatopora tuberculata. Its surface is crowded 

 with rather close-set, rounded papilli, somewhat differing in size. 

 Occasionally an arrangement of somewhat compressed, elongated 

 papilli, in radial order towards certain centres, is faintly indi- 

 cated, but rarely becomes obviously developed. The lamellte are 

 stout, formed of a compact tissue-mass, in which distant perfora- 

 tions are observable, maintaining communication between the 

 superimposed interlaminar spaces. Five or six lamellae occupy 

 the space of one line, interstitial cell-spaces comparatively large, 

 oval or elongated to twice the amount of their height. The 

 course of the laminte is generally in straight parallel lines, but 

 we can often observe their upward inflection at the base of some 

 of the pillars. This produces in vertical sections of this form a 



