On the Relations of Hybocrinus, Baerocrinus, and Hybocystites

Abstract

The Crinoids from the Lower Silurian of Canada for which the genus Hybocrinus was established by Billings in 1856 are remarkable for the protuberance of the anal side of the cup, so that it has somewhat of a humpbacked appearance, as is expressed in Billings's generic name. Resting on the stem are five pentagonal basals, above and alternating with which is a row of five plates (fig. I. a, and Pl. XI. figs. 3, 4, 5). Four of these are arm-bearing radials. The fifth is a hexagonal “anal,” or (as I prefer to call it) azygos plate, on the right upper edge of which rests a small arm-bearing radial, while its left upper edge bears a smaller and true anal plate. These characters are well shown in fig. I. a, which is a copy of Billings's original diagram of the genus; and also in Pl. XI. figs. 3, 4, and 5, which represent the left, right, and anal sides of a specimen from the upper part of the Trenton beds of Mercer County, Kentucky, U. S. A.