These Middle Silurian Period (428-423 mya) horn corals are from the Louisville Limestone of Jefferson County, Kentucky. They appear to be Schlotheimophyllum (referred to by William Davis as Ptychophyllum). Usinig Erwin Stumm's book Silurian and Devonian Corals of the Falls of the Ohio, these horn corals might be Schlotheimophyllum fulcratum (aka by Davis as Ptychophyllum stokesi) or Schlotheimophyllum ipomaea. Another author Herzer in 1902 referred the genus as Chonophyllum.
Stumm refers to these as scattered hollow spines (p.25) when describing the Schlotheimophyllum fulcratum but I believe they are some sort of holdfast or anchor device used by the coral to connect itself to the sea floor/object or another horn coral.
These last three images are of plates from Davis's monograph that have been photo enhanced and labeled. These horn coral specimens were labeled Ptychophyllum stokesi of the Upper Niagara Limestone [now called Louisville Limestone]. Images from book: Kentucky Fossil Corals - A Monograph of the Fossils Corals of the Silurian and Devonian Rocks of Kentucky by William J. Davis, Kentucky Geological Survey 1887. Thanks to Kentucky Geological Survey website of Plate 105.
