PROP. E. W. CLATPOLE ON HELICOPORA. 35 



Outer surface poriferous. 



Rays straight, 50 in an inch ; outer or front face not visibly 

 ridged or keeled, inner face concealed in the matrix. 



Dissepiments on front face rounded, of equal height with the rays, 

 but rather narrower, widening toward the junction; inner face 

 concealed. 



Fenestrules 40 in an inch, varying somewhat in size. Towards the 

 base not extending through the polyzoary, and gradually disappear- 

 ing toward the axial edge ; outer face oval, inner face concealed. 



Pores circular, usually three on each side of a fenestrule, rather 

 more than their own diameter distant from one another. 



The above description of the rays, dissepiments, and fenestrules 

 must be regarded as relating to the lowest portion of the polyzoary, 

 which alone is preserved in the specimen described. 



It was found at Litchfield, Kentucky, by Mr. E. O. Ulrich, of 

 Cincinnati, in beds of Lower Carboniferous age — the Kaskaskia 

 group or Upper Archimedes Limestone. 



This species, H. archimediformis, forms a very interesting term 

 in the series here described. Of nearly the same size as H. Ulrichi, 

 it yet shows a marked advance towards the forms with a strong 

 straight axis which constitute the genus Archimedes of Lesueur. 

 In H. latispiralis, of the Upper Silurian (Niagara group), scarcely 

 the slightest trace of an axis or even of any thickening of the inner 

 border of the polyzoary can be discovered, but the foliate portion is 

 enormously expanded. In H. Ulrichi, from the Lower Devonian, 

 the inner edge is considerably thickened, but does not constitute any 

 thing resembling an axis. In H. archimediformis, from the Lower 

 Carboniferous, the inner edge of the spiral polyzoary is so thick that 

 it approaches somewhat in appearance to an axis, and were it 

 straight might fairly be regarded as such. Centralization, so to speak, 

 had made considerable progress since Silurian days. 



The three species here described prove the continuation of the 

 spiral type of Polyzoon into the Carboniferous, and not its ori- 

 gination there. Two horizons in that system in America have been 

 hitherto the exclusive geological home of the spiral Fenestellids. 

 The Lower Archimedes Limestone or the Keokuk and Warsaw group 

 has yielded 



Archimedes Owenanus, Hall, 



A. reversus, Hall, 



A. Wortheni, Hall 



(if, indeed, A. reversus be entitled to the rank of a species) ; and the 

 Upper Archimedes Limestone or Kaskaskia group has yielded 



Archimedes laxus, Hall, 

 A. Meekanus, Hall, 

 A. Swallovanus, Hall. 



These six (five ?) species are all that have been hitherto described, 

 the genus being at present strictly limited to these two horizons. 



It may appear to some, from the drawing, that the last of the 

 three species here described, H. archimediformis, might be almost 



d2 



