LowEE Helderberg Fauna. 303 



almost perfect accord, and it is in the size and shape of the 

 coralla that the most striking differences are shown. Both 

 forms are covered with a strongly wrinkled epitheca. In F. 

 Helderhergicv the mural pores are said to be in one or two ranges, 

 comparatively large, circular, with margins distinctly elevated ; 

 the tabulae frequent, from ten to fifteen in a space of 10 mm. 

 In F. conicus the mural pores are comparatively large, circular, 

 with distinctly elevated margins, disposed in one, two, or some- 

 times three rows. The tabulae vary from eight to ten in a space 

 of 10 mm. The septa in both forms are represented by longi- 

 tudinal ridges. Hall describes the walls of F. HelderhergicB as 

 sometimes granulose or spinulose on the inner face. In F. conicus 

 the interior of the cells is said to give evidence of numerous 

 spinules or small nodes. 



Of F. conicus^ Prof. Hall says, ^' The conical form of this species 

 and the irregularity in the size of the cells distinguish it from every 

 other known species of Favosites in the Silurian rocks of New 

 York.""^ In the same description the diameter of the cells at 

 the surface is said to vary from 1.5 to 3.5 mm. This places 

 their size as a little greater than in F. Helderhergioe^ where they 

 range from . 66 to 2 mm. My own measurements show the aver- 

 age size of the corallites in either species to be from 1.5 to 

 2 mm., and in F. conicus from about . 5 to 3 mm. The collection 

 at my command probably does not present the range of speci- 

 mens which Prof. Hall studied. It is possible that Favosites 

 conicus and F. Helderhergim both refer to the same organism 

 at different stages of growth and preservation. F. Helder- 

 hergicB is found in and at the top of the Lower Pentam- 

 erus limestone, in. the neutral zone between that and the 

 Shaly limestone. F. conicus is obtained from the Shaly lime- 

 stone itself, but, toward the base of that horizon, the specimens 

 occur silicifled, and of a larger size, thus displaying a great 

 resemblance to F. Ilelderhergice. The conditions of argillaceous 

 sedimentation are unfavorable to the growth of corals, and it 

 is possible that F. conicus simply represents immature or 

 small forms of the larger species, which a change from favorable 

 to unfavorable conditions has uniformly prevented from further 

 growth. 



* Hall, 1SS7. Pal. New York, vol. VI, p. 9. 



