MINUTE STRUCTURE OE STROMATOPORA AND ITS ALLIES. 215 



dimensions, forming flattened elliptical masses from six inches to 

 a foot in diameter ; and tbe same form is found in tlie Niagara 

 Limestone in Canada *. When superficially examined, this species 

 is almost undistinguishable from Stromatopora striatella, D'Orb., 

 which it closely resembles in the extreme closeness and density 

 of its reticulate tissue. When examined, however, by means of 

 thin vertical sections, this form is found to differ fundamentally 

 from S. striatella in not consisting of distinct horizontal laminse, 

 separated by interspaces crossed by radial pillars. On the con- 

 trary, the entire structure is minutely vesicular, like a GystipJiyl- 

 lum, but much more delicate, composed of innumerable oval vesi- 

 cles placed in concentric lines, and very often opening into each 

 other at both extremities. In other words, the horizontal laminse 

 are inflected so as to form a number of small oval cavities, which 

 may or may not be separated from one another ; and when these 

 inflections are so pronounced as actually to separate contiguous 

 laminse, then they represent the " radial pillars " of the normal 

 Stromatoporoids. 



Another form belonging to this group is a large massive Stro- 

 matoporoid from the Corniferous Limestone of North America, 

 of which we possess numerous well-preserved examples, all of 

 which, however, are infiltrated with silica, the skeleton itself 

 generally remaining calcareous. In this form the minute struc- 

 ture is essentially similar to that of the preceding, but on a 

 comparatively gigantic scale. The entire mass is composed of 

 oval or elongated cells with calcareous walls, generally about 

 three to a line, placed in horizontal rows, and sometimes opening 

 one into the other by wide apertures. Though the longitudinal 

 apposition of the cells in concentric and parallel rows gives the 

 appearance of there being distinct horizontal laminse, each vesicle 

 has its own proper walls as a rule, as is distinctly shown in many 

 instances where two vesicles are placed end to end without their 

 walls coalescing. The vesicles, however, may be regarded as 

 produced by the rapid undulation and inflexion of the horizontal 

 laminse, the " radial pillars" not existing as distinct structures. 

 Horizontal sections show a similar network of oval vesicles 



* We have recently found a precisely similar form to be of quite common 

 occurrence in the Wenlock Limestone of the West of England ; and though we 

 have not yet been able to make thin slices of this, we are led to think it possible 

 that the form usually termed Sfromatopora striatella, D'Orb., may really be 

 none other than this. 



