S UB- GENUS MONO TR YPA . 



173 



two inches, the calice-bearing upper surface being nearly flat. 

 Tangential sections (fig. 32, r.) show that the ordinary corallites 

 have an average diameter of from i-7oth to i-5oth inch, while 

 at the angles of junction of these are often small angular tubes 

 of I -300th inch diameter or less. The angles of junction of 

 adjoining corallites are often slightly thickened, though never 

 conspicuously so, and the corallites are otherwise uniformly 

 thin-walled. 



On the other hand, the specimens from the Hudson River 

 Group (fig. 33, a) form rounded or irregularly spheroidal 



Fig- 33- — A, A specimen of MonticuHpora iimiidata, Nicli., from tlie Hudson River Group of 

 Canada, of the natural size, partly broken away on one side ; B, Tangential section of the 

 same, enlarged eighteen times ; c, Vertical section of the same, similarly enlarged, show- 

 ing the wavy walls and the remote tabula;. 



masses, generally from an inch to an inch and a half in dia- 

 meter, and sometimes growing round the stem of a Crinoid. 

 None of my specimens exhibit an unworn surface. Tangen- 

 tial sections (fig. 'i,'^, b) show that the ordinary corallites have 

 an average diameter of from i-QOth to i-6oth of an inch, and 

 that they are therefore, upon the whole, slightly smaller In 

 their dimensions than is the case with the corresponding 

 corallites in the Trenton Limestone specimens. A consider- 

 able number of small angular corallites, of an average diameter 

 of I -300th inch, are wedged in at the corners of the larger 

 tubes. As a rule, the corallites in tangential sections exhibit 

 the same thin-walled and delicate structure that Is characteristic 

 of the Trenton Limestone examples ; but In some parts of these 

 sections (probably the parts nearest the original surface) the 

 walls become decidedly thickened, and very conspicuous nodes 



