60 



THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



and that the cirri, which in dicychc 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 : — 



1. 





Dicyclic. 



Monocyclic. 



Basals. 



Interradial. 



Interradial. 



2. 



Infrabasals, 



Radial. 





3. 



Column. Exterior angles of. 



Interradial. 



Radial. 





Sections of. 



Interradial. 



Radial. 



4. 



Column. 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 Palaeozoic 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 dicyclic Palgeocrinoids. Though universally considered to be monocycHc 

 Crinoids, they were built on a dicyclic plan ; and this led us to inquire 

 whether all those forms might not be dicychc, whose infrabasals were hidden, 

 or had existed in their larval state. 



This seemed to be confirmed by another observation which we made 

 among certain Palaeozoic forms. In some genera, notably in Graphiocrinus, 

 which was originally described as monocychc, 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 

 Milkricnmis, Apiocrinus, and Fentacrinus, for the stem occupies the same posi- 



