RELATIONS TO EXTINCT ORGANISMS. 



89 



I do not myself know of any Monticuliporoid which could 

 be compared with Heterodictya except M. pavonia, D'Orb. : and 



Fig. 15. — Minute structure o^ Hdcrodictya gigantea, Nich. , from the Corniferous Lime- 

 stone (Devonian) of Canada. A, Vertical and longitudinal section, showing the well- 

 developed tabulre ; B, Vertical and transverse section, showing the incomplete condition 

 of many of the tabulre ; c, Tangential section. All tlie figures are enlarged eighteen 

 times. 



this is, in my opinion, sufficiently separated from the former by 

 its want of any definite form, or of definite and structurally 

 differentiated margins ; by the absence of a complete median 

 calcareous lamina, which is so clearly a distinct structure that 

 the layers of tubes can be readily stripped away from it ; and 

 by the possession of distinct '* monticules." The tabulae of 

 M. pavonia, D'Orb., are even more feebly developed than in 

 Heterodictya ; but this is a feature of no importance — whereas 

 the striking difference in general aspect, in the want of obli- 

 quity to the calices, in the permanent distinctness of the walls 

 of adjoining tubes, and in the structural characters above 

 noted, sufficiently prove that M. pavonia cannot, at any rate, 

 be referred to the genus Heterodictya. 



