I20 LOWER PENINSULA. 



Ptychophyllum Stockcsii says nothing about such excrescences, 

 but otherwise it applies exactly to the specimens now considered, 

 which were found in the same locality as his, at Drummond's 

 Island, associated with the other form. Other forms of Ptycho- 

 phyllum, found at Louisville, in the Niagara group, and resembling 

 PtycJiopJiylliim patellatiim from GotJiland, have these root-like ap- 

 pendices also well developed. It is sometimes difficult to draw a 

 line of distinction between specimens of these two species. 



Plate XLIV., upper tier, gives various silicified specimens from 

 the Niagara group of Drummond's Island. 



A very common species in the Niagara group of Iowa, described 

 by D. Dale Owen, under the name of CyatJiopJiylhini nndiilatiun et 

 vmltiplicatum, has a structure entirely conformable with the two 

 species of Omphyma of Drummond's Island, and must therefore be 

 arranged with them in the same generic group. 



DIPHYPHYLLUM, Lonsdale. 

 Synon., Eridophyllum, Milne-Edwards. 



DiPLOPHYLLUM, Hall. 



Colonies of aggregated cylindrical polyp cells, multiplying by caly- 

 cinal gemmation, but not by fissiparous mode of propagation, as 

 Lonsdale asserts. The stems are rarely in intimate contact so as 

 to form astraeiform masses ; usually some interval remains between 

 them, and they are mutually connected by rugose or radiciform 

 lateral prolongations of the walls, or by floors formed by periodical 

 horizontal expansions of the calyx margins until they join at their 

 edges. Structure of cells biareal. The outer area is formed by the 

 external epithecal wall with a cycle of stout vertical lamellae having 

 crenulated edges and arched carinae decorating their flanks. The 

 interlaniellar interstices are filled with small transverse plates, divid- 

 ing them into narrow cellulose spaces. These transverse plates are 

 disposed in arched rows, crossing the arched carinae diagonally 

 from within and below, upwardly and outwardly. The inner area 

 is principally composed of flat, transverse diaphragms, which are 

 only in their circumference intersected by the radial lamellae ; their 

 centre is free of the crests, or the crests extend only on their upper 



