20 LOWER PENINSULA. 



phragms, either simple, or compound through the anchylosis of 

 several plates meeting at angles, or rendered imperfect by partial 

 plates not anchylosed. Tube cavity exhibiting twelve longitudinal 

 furrows, and having the band-like intermediate spaces frequently 

 decorated with one or several vertical rows of spinules, or with a 

 row of horizontal, squamiform leaflets. These characters of radi- 

 ation arc, however, not in all species equally distinct, becoming in 

 some specimens nearly obsolete, while in others of the same kind 

 they may be exhibited in obvious development. The rows of 

 squamose projections, peculiar to certain Devonian forms, by com- 

 ing in contact and adhering together at their joined edges, form 

 compound, perfect, or incomplete diaphragms, interposed between 

 the regularly formed simple diaphragms, and in some specimens 

 these prevail to such an extent as to altogether exclude the regular 

 diaphragms. 



Milne-Edwards proposed for Favosites of the above indicated 

 structure the name Eninionsia, but the character of the dia- 

 phragms in the forms in question is so variable and inconstant, 

 that one and the same specimen, or even the same identical tube 

 channels, may at a certain period of growth be intersected by com- 

 pound complicated diaphragms, exhibiting at another only sim- 

 ple, straight diaphragms like other forms of Favosites, while in 

 the prolongation of the tubes they may be divided again by com- 

 pound or imperfect septa. A character so changeable and peculiar 

 to almost every one of the Devonian species of Favosites can not 

 be used as a generic distinction for those which have it in a more 

 marked degree than the others. Milne-Edwards has also con- 

 fused with his Emmonsia a species which stands in no special rela- 

 tionship to the other forms ; his Emmonsia cylindrica is a true 

 typical form of a MicJielinia. 



The genus Favosites and the whole family of Favositoids had 

 comparatively a very brief period of existence. The first forms 

 appear in the upper Silurian deposits (Clinton group). During the 

 Niagara period and the Devonian era, Favositoids abounded, and 

 were the prevailing representatives of coralline life. The carbon- 

 iferous strata inclose only a few of these forms, and in later depos- 

 its no representative of the genus is known. The Silurian forms 

 differ from the Devonian Favosites by invariably having simple dia- 

 phragms, and by the spinulose character of their radial crests. The 



