SUB-GENUS DIPLOTRYPA. 167 



tubes approach each other, are numerous " spinlform coralHtes," 

 which agree with the same structures in other forms of the 

 genus in having a small circular central cavity, surrounded by 

 exceptionally dense dark-coloured walls. In this case, how- 

 ever, these peculiar tubules have generally a more or less 

 quadrangular outline. The upward terminations of these on 

 the surface are unknown. 



Vertical sections (PL IV. fig. A^b) show the extreme thinness 

 of the corallum, and the shortness of the corallites. The two 

 sets of ordinary corallites are now quite easily recognised, both 

 by their different sizes, and also by the marked difference in 

 their internal structure. The large corallites do not seem to 

 be provided with tabulae, — at least I have not succeeded in 

 certainly detecting any, — but their cavities often show a funnel- 

 shaped inward extension of the matrix along the axis of the 

 visceral chamber. The small angular corallites, on the other 

 hand, are provided with numerous, complete, and horizontal 

 tabulae. Occasionally, also, a few of the thick-walled tubuli 

 may be recognised in sections of this nature. 



The forms with which one would be disposed, upon the 

 basis merely of external features, to compare J\L calycttia, 

 James, are M. Newbcrryi, Nich., and M. discoidea, James. 

 The present species, however, differs entirely in its internal 

 structure from either of the types just mentioned ; and I know 

 of no species which is structurally so nearly allied to it as to 

 demand a close comparison. 



The specimens upon which the above description is founded 

 were collected by Mr U. P. James of Cincinnati. 



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



