60 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
and that the cirri, which in dicyclic Crinoids are radial, are interradial in the 
others. By employing this rule we were enabled, in many cases where the 
infrabasals are hidden beneath the column, to determine their presence. The 
law of alternate arrangement of the successive parts is shown by the follow- 
ing table : — 
Dicyclic. Monocyclie. 
Basals. Interradial. Interradial. 
Infrabasals. Radial. 
Column. Exterior angles of. Interradial. Radial. 
Sections of. Interradial. Radial. 
Sutures. Radial. Interradial. 
Sides. Radial. Interradial. 
Cirri, when present. Radial. Interradial. 
Axial canal. Radial. Interradial. 
This law is only applicable, to its full extent, in species with pentangular 
or pentapartite stem and canal; but we infer, from analogy of these forms, 
that the circular stem, wherever it occurs in dicyclic Crinoids, is also practi- 
cally interradial, and in those of monocyclic type radial. Our observations 
were founded upon Paleozoic Crinoids, among which we had met with no 
exception; but on applying them to Mesozoic and later Crinoids, we were 
surprised to find that in most of the so-called monocyclic forms the relative 
positions of the column and cirri were reversed, so that they were exactly as 
in dicyclie Paleocrinoids. Though universally considered to be monocyclic 
Crinoids, they were built on a dicyclic plan; and this led us to inquire 
whether all those forms might not be dicyclic, whose infrabasals were hidden, 
or had existed in their larval state. 
This seemed to be confirmed by another observation which we made 
among certain Paleozoic forms. In some genera, notably in Graphiocrinus, 
which was originally described as monocyclic, the infrabasals are so extremely 
small that they are completely covered by the upper stem joint, and only the 
basals are visible. These Crinoids are practically in the same condition as 
Millericrinus, Apiocrinus, and Pentacrinus, for the stem occupies the same posi- 
