146 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
While adhering, therefore, to the original definition of Miller for the 
Crinoids, and recognizing the Blastoids and Cystids as distinct groups, of 
relatively equivalent rank, we have to admit that the three types are con- 
nected by a number of remarkable intermediate forms, and that it is ex- 
tremely difficult in many cases to ascertain whether certain forms are Crinoids 
or Cystids, or Cystids or Blastoids. It has been stated that Crinoids and 
Blastoids are distinguished from Cystids by their distinct pentamerous sym- 
metry; that the Blastoids and Cystids, as opposed to the Crinoids, have no 
true arms; that the Blastoids have hydrospires, the Cystids calycine pores, 
elc.; but the best of these characters meet with exceptions. We find in 
certain Blastoids, and also in Tiaracrinus from the Devonian, and Zophocrinus 
from the Upper Silurian, only four radii; and even as late as the Mesozoic 
there are among the Plicatocrinide species with three, four, and six rays. 
Porocrinus has the calyx and arms of a Crinoid with calycine pores of a 
Cystid. Hybocystis has stumps of arms with recurrent ambulacra on three 
of its rays, and on the other two rays calyx ambulacra, which pass down 
the cup as far as the basals. The genus was described by Wetherby as a 
Cystid, by Etheridge and Carpenter as a transition form between Crinoids 
and Blastoids; while we have regarded it a Crinoid with strongly persist- 
ent Cystidean characters. Caryocrinus, which has always been considered 
a Cystid, has segmented pinnule-bearing arms like a Crinoid; but it has 
calycine pores and hydrospires, and according to Carpenter six rays. Codaster 
was made a Cystid by E. Billings, by Etheridge and Carpenter a Blastoid. 
Stephanocrinus was placed by Roemer among the Cystids, by Etheridge and 
Carpenter among the Blastoids, and we take it to be a Crinoid. Asteroblastus 
has calycine pores like a Cystid, and ambulacra and pinnules like a Blastoid. 
These, and other facts that might be adduced, point to a common origin; 
but what may have been the exact line of derivation between the three 
groups is a problem that is difficult to solve. They are found side by side 
in the Lower Silurian; but while the Cystids ceased to exist at the end 
of the Devonian, and the Blastoids at the close of the Carboniferous, the 
Crinoids survived to the present day. From this it seems to be evident that 
the Cystids, as the lowest in rank and earliest in time, were the ancestral 
type, and the progenitors of the other two; but it is possible that the Crinoids 
preceded the Blastoids, although the latter became extinct before the close 
of Paleozoic time. 
That the origin of the Crinoids must have dated back far beyond the 
