312 TABULATE CORALS. 



separates the species from F. minor, M'Coy, this latter Hke- 

 wlse having a much greater development of the interstitial 

 corallites. 



Formation and Locality. — Common in the Hamilton Group, 

 Canandaigua, State of New York. 



Sub-genus Diplotrypa, Nich., 1879. 



I propose this name as a subgeneric designation for a group 

 of MonticuliporcE, of which M. petropolitana, Pand., is the 

 type. The corallum in the forms in question is like that of 

 the sub-genus Heterotrypa in being conspicuously dimorphic ; 

 but all the corallites are markedly angular or pris7?iatic, while 

 their walls are remarkably thin and delicate, and their general 

 appearance resembles that of the corallites of Favosites. The 

 large corallites have few and remote tabulae, and are usually or 

 always aggregated at special points into clusters or " monti- 

 cules ; " but they are also scattered throughout the entire 

 colony, the bulk of which they compose. In the "monticules" 

 the large tubes are usually alone present, but in the other parts 

 of the corallum the larger corallites are partially separated by 

 the development of a larger or smaller number of small coral- 

 lites, which agree with the large ones in being angular in shape, 

 but differ in being provided with much more numerous and 

 close-set tabulae. The characters of the group will be best 

 elucidated by a brief consideration of the internal structure of 

 M. {piplotrypd) petropolita7ia^ Pand. 



