CARBONIFEROUS FOSSILS. 



more acutely conical, central protuberance ; while it presents differences in the 

 details of its internal structure. As in that species, a vertical section shows its 

 outer vesicular area to be comparatively thin, and composed of very small vesicles, 

 the middle zone between this and the pseudo-columella of coarser, transverse 

 vesicles ; and the false columella itself of elongated, slanting vesicles. In our 

 species, however, the comparative breadth of these zones is different ; its middle 

 one, of transverse vesicles, between the false columella and the outer area, being 

 proportionally much broader, or nearly equalling the breadth of the pseudo- 

 columella ; while, according to Prof. McCoy's figures, the columellar space is ten 

 to twelve times as broad as the middle zone in his species. Again, the outer vesi- 

 cular zone in the species under consideration is proportionally a little broader 

 where fully developed. 



It is worthy of note that the outer area is merely rudimentary in small specimens, 

 and probably entirely wanting in very young examples of our species. In speci- 

 mens of one inch to one and a quarter inches in length there is usually but a single 

 series of these small outer vesicles, rising like little blisters on the inner side of the 

 epitheca between the rays. Farther up from the smaller extremity of the corallite 

 another alternating series is developed upon these, and still higher another, and 

 so on until ten or twelve series are produced in large specimens ; beyond which 

 there would seem to have been no farther increase. The vesicles themselves of 

 the outer zone appear to be of uniform size at all stages of growth ; their height 

 being about 0.04 inch, and their breadth about 0.02 inch. No well-defined walls 

 were observed separating the outer and middle zones, nor the latter from the 

 pseudo-columella. 



The specific name is given in honor of Mr. W. M. Gabb, Palaeontologist of the 

 State Geological Survey of California. 



VOL. I. — I 



