26 LOWER PENINSULA. 



Favosites hcmisphericus is one of the best marked species of 

 Favosites ; it has a wide range of variations, but its pecuHar mode 

 of growth makes it easily distinguishable from any other form. 



The tubes of this form are about two millimeters in diameter, of 

 unequal size, rounded-polygonal ; tube cavity generally smooth, 

 intersected by simple flat diaphragms. It occurs rarely that the 

 diaphragms arc compound and angular on the surface, formed by 

 anchylosis of lateral, squamiform projections. Lateral pores large, 

 usually in a single row on each side, and moderately distant. 

 Sometimes, however, two rows of pores may be observed on a s^e. 

 The mode of growth mentioned as the most characteristic feature 

 of this species is nevertheless quite variable. We find polyparia 

 of subspherical or of biconvex lenticular form, or in cylindrical, 

 irregularly flexuose, root-like masses, over a foot in length, or in 

 elongated horn-shape, all of which forms proceed from a single 

 proliferous mother-tube. At first the polyparium is attached by its 

 narrow and usually excentrical apex, but soon it becomes free, and 

 the apex is folded over by the spreading margins of the rapidly 

 enlarging corallum. The tubes diverge in graceful curves from an 

 imaginary central axis toward the periphery. Those ends, ter- 

 minating on the lateral faces of the corallum, have their walls 

 thickened in their peripheral portion, and their orifices are all 

 closed by opercula of concentric annular structure, with a central 

 opening while growing, which is finally closed by a solid nodular 

 piece. The margins of the opercula are frequently decorated by 

 twelve carinse converging from the margins toward the centre, but 

 not reaching it. In specimens with excessively thickened wall 

 substance, these radial carinse are very obscure, or entirely obliter- 

 ated. The orifices terminating on the convex disk of the corallum are 

 all open, more thin-walled than the others, and of more pronounced 

 polygonal form. It often happens that these centrally situated, 

 thinner-walled tubes have been destroyed by weathering, while the 

 exterior lateral tube ends, of massive structure, have resisted and 

 been preserved. The upper end of such specimens is deeply exca- 

 vated, and the lenticular forms are transmuted into concave, pa- 

 telliform dishes. The elongated, horn-shaped specimens termi- 

 nate in this case with a funnel-shaped excavation resembling the 

 calyx of a Cyathophyllum, which resemblance is augmented by 

 the exposure of the side faces of the septate tubes, arching from 



