72 LOWER PENINSULA. 



lose prominences. This structure is in general conformity with the 

 structure of Favosites, from which it differs in the vesiculose nature 

 of the diaphragms, and in having a much greater number of longi- 

 tudinal furrows. In Favosites these never exceed the number 12, but 

 in Michelinia they are more than double that number. The tubes 

 of Michelinia are usually also of much larger size than in Favosites. 

 The genus Michelinia appears nearly contemporaneous with Favo- 

 sites in the upper Silurian strata. A small nummiform species, 

 which to my knowledge has not yet been described, occurs in the 

 Niagara group of Tennessee. Another somewhat larger but very 

 similar form is found in the lower Helderberg strata of Schoharie 

 County, N. Y., which is described by Mr. Hall under the name of 

 Michelinia lenticularis. None of these have been found in Michi- 

 gan. The Devonian formation contains a variety of forms, nearly 

 all of which are represented in Michigan. Formerly the casts of 

 small specimens of Michelinia were through misapprehension classed 

 under the name of Pleurodictyum problematicum, as being a fossil 

 of peculiar organization. I have exposed this error in an article 

 published in 1862 in Silliviaii s Joiirnal. 



MICHELINIA CONVEXA, D'Orbignv. 



Hemispherical masses, with a depressed, turbinate, discoid under 

 side, covered by a concentrically wrinkled epitheca, formed of di- 

 verging, large, conical tubes of unequal size and of rounded-poly- 

 gonal outlines, the larger ones attaining a diameter of one centi- 

 meter ; walls stout. Transverse diaphragms globoso-convex, com- 

 posed of larger and smaller irregularly interlacing vesiculose plates. 

 Vertical furrows in the circumference of a tube about forty, with 

 intermediate rows of short, spinulose projections. The surface of 

 the diaphragms is generally smooth ; in rare instances they are cov- 

 ered with granular prominences. Pore openings small, surrounded 

 by a projecting rim, irregularly dispersed, and in some parts much 

 more crowded than in others. Found in the drift, associated with 

 corniferous limestone fossils in silicificd condition. Very common 

 in the corniferous limestone of Port Colborne, C. W. 



Plate XXVI. — P^ig. i, view from above ; P^ig. 2, view of the lower 

 side of silicified specimens from Port Colborne. 



