192 THE GENUS MONTICULIPORA. 



Pand., and, in fact, possesses peculiar characters which suffici- 

 ently distinguish it from all the forms externally like it. Tan- 

 gential sections (fig. 40, b) show that there is a total absence 

 of small interstitial tubuli, and that, except for the previously 

 mentioned existence of clusters of corallites of a size slightly 

 above the average, the tubes are approximately alike in size. 

 The walls are exceedingly thin, but nevertheless, in sections 

 sufficiently thin, show a distinctly duplex character, the bound- 

 ary lines between the walls of contiguous tubes being expanded 

 into angular nodal points at their angles of junction {see fig. 2, b). 

 Long sections (fig. 40, c) show that there is no difference in in- 

 ternal structure between the average corallites and the clusters 

 of slightly larger tubes which are interspersed amongst these, 

 all being furnished with numerous complete tabulae. The two 

 species most nearly allied in structure to the present form are 

 ]\L nndnlata, Nich. (the rounded examples from the Hudson 

 River Group), and M. irregularis, Ulrich. From the former 

 of these, M. pctasiformis is distinguished by its different 

 external shape, by the fact that its corallites spring perpen- 

 dicularly from a basal epithecal plate, instead of radiating from 

 a basal or central point, by the absence of even a few small 

 corallites, and by the much greater development of the tabulae, 

 these structures not being disposed at corresponding levels in 

 contiguous tubes. From ]\I. irreg2daris, Ulrich, the present 

 species is readily separated by its want of the nodulated sur- 

 face and radiated corallites of the former, and by the great 

 abundance of its tabuke. 



The specimens of AT, pctasiformis upon which I have 

 founded the species were kindly furnished to me by Mr John 

 M. Nickles, of Cincinnati, who has devoted much time to the 

 collection and study of the corals of the Cincinnati Group. 



Horizon and Locality. — Cincinnati Group, Ohio. 



