SUB-GENUS MONOTRYPA. 187 



walled and polygonal, and of tolerably equal dimensions (PI. I. 

 fig. 3<^). As regards internal structure, tangential sections 

 (PI. I. figs. 3<r and '^d) show that the ordinary polygonal coral- 

 lites have a few smaller ones intercalated amongst them — these, 

 however, being shown to be only young tubes by the fact of 

 their having no special peculiarities of internal structure, and 

 by their inconstant development. On the other hand, in care- 

 fully prepared tangential sections, I find a number of small 

 triangular or sub-circular tubes to be present at the angles of 

 junction of the ordinary corallites (PI. I. fig. 3^^). These do 

 not seem to be bounded by definite walls of their own ; and as 

 I have wholly failed to detect their existence in longitudinal 

 sections, I am inclined to think that they are not really coral- 

 lites or interstitial tubules, in any proper sense, but merely 

 vacuities in the wall, such as undoubtedly occur in some allied 

 types of corals. Longitudinal sections (PI. I. fig. 3^) show 

 that the corallites spring nearly at right angles to their base 

 of origin, and follow a tolerably straight course to the sur- 

 face, their thin walls being slightly thickened as they approach 

 their termination. Tabulae, which, I believe, are stated to be 

 absent by Messrs Miller and Dyer, are, so far as I have seen, 

 well developed, being always complete and approximately 

 horizontal, and increasing in number as the calicine surface 

 is approached. 



I am indebted for the specimens upon which the preceding 

 description has been founded to Mr John M. Nickles, of Cin- 

 cinnati, who has specially devoted himself to the study of 

 these minute and difficult Palaeozoic fossils. 



Horizon and Locality. — Cincinnati Group, Cincinnati, Ohio. 



