SUB- GENUS PRASOrORA. 2ti 



ences. The large tubes are always doubly tabulate, one set 

 of tabulae forming a series of large lenticular vesicles, the con- 

 vex sides of which are directed inwards towards the centre of 

 the visceral chambers, while the remaining tabulae are horizon- 

 tal and remote, and extend from the lateral wall of the coral- 

 lite to the inner margin of the above-mentioned vesicles. In 

 some of the tubes we may occasionally notice the convex 

 tabulae to form isolated vesicles, as they usually do in the 

 specimens from the Trenton Limestone ; but they are more 

 commonly so apposed to one another as to form vertical rows 

 of lenticular cells, the inner margins of which unite so as to 

 constitute an apparent median septum to the corallite. The 

 small corallites are uniformly furnished with numerous com- 

 plete, horizontal tabulae. Upon the whole, I have little doubt 

 that the specimens now described from the Cincinnati Group 

 of Ohio are not specifically separable from the true M. Sel- 

 wynii of the Trenton Limestone, 



In form and general aspect, M. Sclwynii closely resembles 

 such types 2S M . petropolitana , Pand., or A4. Whiteaveszi, Nich. 

 In its internal structure, however, it is clearly allied to quite 

 another group of types — namely, to such forms as M. Cincin- 

 natiensis, James, M. froiidosa, D'Orb., Jll. Ncivberjyi, Nich,, 

 and M. molesta, Nich., all of which agree with Prasopora 

 Grayci", Nich. and Eth. jun., in the possession of a peculiarly 

 modified structure of the tabulae of the larger corallites. In 

 fact, M. Selwynii is more nearly allied to Prasopora Grayce, 

 Nich. and Eth. jun., than to any other type known to me, and 

 it must be regarded as a member of the sub-genus Prasopora, 

 as I have previously defined it. In its purely specific char- 

 acters, however, it is so far distinct that it requires no close 

 comparison with any other form with which I am acquainted. 



Horizon and Locality. — In the Trenton Limestone of Peter- 

 boro', Ontario (the type-form). In the Cincinnati Formation 

 of Waynesville, Ohio (var. hospitalis). 



