54 LOWER PENINSULA. 



specimens, a cycle of longitudinal carinations is faintly visible. In 

 its structure this elegant small species approaches Favosites limi- 

 taris, which could likewise be not inappropriately arranged under 

 the genus Cladopora. The thick-walled conical tubes, the greater 

 obliquity of the orifices to the surface, with a sometimes well-de- 

 veloped prominent lip, bring this form nearer to the Cladopora type 

 than to Favosites. Very common in the drift boulders of the cor- 

 niferous formation in Michigan ; it occurs in place in the Helderberg 

 limestones of Canada, and at the Falls of the Ohio, near Louisville. 

 Plate XXI. — Fig. i represents a number of variations, amongst 

 which are found stems with smaller tubes and with larger tubes, 

 with projecting monticulose orifices and with lipped mouths, or 

 with massive surface of the stems, with the orifices impressed as 

 shallow pits. Some of the stems represented exhibit also sharp, 

 linear, polygonal furrows circumscribing the tubes. 



CLADOPORA ROBUSTA, N. Sp. 



Palmato-ramose, occasionally reticulated stems, of round or com- 

 pressed elliptical form, growing in horizontal expansions, spreading 

 sometimes over the space of several square feet. Stems attaining a 

 thickness of from one to two centimeters. Tube openings oblique 

 to the surface, with gently dilating orifices, joining under subacute 

 margins, by which the surface is divided into a network of rhom- 

 boidal spaces with rounded corners. The lower angle of the rhom- 

 boids is formed by the projecting semicircular lips of the orifices, 

 which, by wearing off, become emarginated and acutely triangular 

 in shape. Transverse diameter of orifices about one millimeter, 

 in other varieties smaller. Diameter of tube channels below the pe- 

 ripheral surface one quarter to one half of a millimeter. Tube walls 

 stout, thickening near the periphery. Lateral pores distant. Dia- 

 phragms sometimes observed, but not developed in the majority of 

 the specimens. Several varieties, in manner of growth and size of 

 tubes, can be distinguished. 



Found in the corniferous limestone and in the Hamilton group. 

 The Hamilton specimens occur in the vicinity of Alpena, on Thun- 

 der Bay ; the specimens of the corniferous strata are frequently 

 found in the drift of Michigan, but the P^alls of the Ohio is the lo- 

 cality where this species can be found in greatest perfection and in 



