6o LOWER PENINSULA. 



Plate XXIII., Fig. 5.— The three upper specimens were found 

 at Thunder Bay, near Alpena ; the three lower specimens are 

 from Widder, Canada West. Fig. 6 of the same plate represents a 

 larger form, found in the corniferous limestone of the Falls of the 

 Ohio, which does not seem to differ much from the Hamilton form, 

 except by a more robust growth, and by less definitely circumscribed 

 orifices. 



DENDROPORA, Michelin. 



The corals, one of [which is described by Michelin under the 

 name of Dendropora, have been placed by Milne-Edwards with the 

 sub-order Seriatoporinae, through misapprehension of their struc- 

 ture, attributing to them a central columella which does not exist. 

 Their structure is in all essential points identical with the Favo- 

 sitinae. Milne-Edwards divides these forms into three genera : 

 Dendropora, Rhabdopora, and Trachypora, according to certain 

 surface characters which I have not considered important enough 

 to justify the separation. In the following pages, therefore, I use 

 the name Dendropora for all of them, and give the subjoined defi- 

 nition of the term Dendropora : 



Branching and frequently reticulated stems, variable in diame- 

 ter from one millimeter to more than one inch. The stems are at- 

 tached to other bodies by an incrusting basal expansion ; they are 

 composed of very thick-walled, intimately united conical tubules, 

 diverging from an imaginary axal centre in ascending curves. The 

 tube channels are laterally connected by pore channels and trans- 

 versely septate by diaphragms. The interior tube ends' are only 

 moderately thick-walled, but in approaching the periphery the walls 

 thicken very much by the addition of concentric layers within the 

 expanding channels, and constitute by their intimate union a 

 broad interstitial surface separating the orifices. In some forms 

 this interstitial surface is covered by spinulose ridges and granula- 

 tions ; in others by flexuose longitudinal rugae, or by a combination 

 of both granules and rugae, with intermediate punctiform or short 

 fissure-like porosities, which are not cell spaces of an independent 

 tissue element, but are merely superficial punctations and engrav- 

 ings of the substance of the tube walls. The orifices usually pro- 

 ject with their margins above j^the general surface, but sometimes 



