GENERA OF CH^ETETID^E AND MONTICULIPORID^E. 319 



and small tubes. The former are not uniformly similar in 

 structure. In some cases they are simply crossed by a few 

 remote tabulae, as is usual among the Monticulipora. In 

 other cases, and more commonly, however, they exhibit 

 throughout or in part of their course a very singular double 

 arrangement of the tabulse, the explanation of the phenomena 

 in question being still unknown to me. In these cases the visc- 

 eral chamber of the corallite may be considered as divided into 

 two lateral halves or compartments. In one half of the tube 

 the tabulae are not only remote, but are strongly convex, with 

 their convexities directed upwards, or towards the centre of 

 the visceral chamber, thus forming a series of large lenticular 

 vesicles, the internal margins of which unite so as to form an 

 apparent median septum to the tube. In the other half of the 

 corallite the tabulae are simply remote, complete, and horizontal, 

 extending from the lateral wall of the corallite on the one hand 

 to the inner margin of the lenticular vesicles just mentioned on 

 the other hand. Lastly, the smaller corallites are uniformly 

 provided with very numerous crowded and complete horizontal 

 tabulae. 



The features which separate M. (Diplotrypci) Whiteavesii, 

 Nich., from the typical M. petropolitana, Pand., have been 

 alluded to in speaking of the latter, and need not be further 

 insisted on here. The internal structure of the corallum, and 

 especially the remarkable form of tabulation displayed by the 

 greater number of the large corallites, will serve to differen- 

 tiate the species from all other types which resemble it in 

 general shape and aspect. 



I have named the species after my friend Mr Whiteaves, 

 the accomplished palaeontologist of the Canadian Geological 

 Survey. 



Formation and Locality.- Abundant in the Trenton Lime- 

 stone of Peterboro', and of other localities in Ontario. 

 Specimens of a rounded form attached by a broad base to 

 foreign bodies occur also in the Cincinnati group of Waynes- 

 ville, Ohio. 



