SUB- GENUS MONOTRYTA. 



171 



mation of Canada, similar in form to some of those included by- 

 Hall under the name of Chcrtctcs lycopcrdou, Say (Geol. Mag., 

 Dec. ii., vol. ii. p. 177). I have now had the opportunity of 

 examining these latter forms microscopically, and I find that 

 I was in error in associating- these with the Trenton Limestone 



Fiy. 32. — ]\IonlictiIipora {Monotrypa) midiilata, Nicli., froin the Trenton Limestone of 

 Canada. A, Part of the type-specimen, in outline, of the natural size; B, Tangential 

 section, enlarged eighteen times, showing the thin-walled angular corallites ; c. Part of a 

 longitudinal section, similarly enlarged, showing the wavy walls and the sparse taljulx. 



species. On the other hand, a minute examination of the 

 corals of the Hudson River Group of Canada, which have 

 commonly been spoken of as " puff-ball varieties of Stcuopora 

 fibrosa',' and which I used to regard [op. jam cif., p. 176) as a 

 mere variety of AT. pctropolitana, Pand., has shown me that 

 these are in all essential respects entirely identical in internal 

 structure with the AL iindulata of the Trenton Limestone, 

 from which they differ principally in their smaller size and 



