SYSTEMATIC PART. 147 
Silurian and perhaps Cambrian age, is indicated by the high state of develop- 
ment which some of their forms had acquired as early as the Trenton 
group, when they had in some cases almost completely thrown off their 
Cystid characters. A striking example of this among the Camerata is seen 
in the genus Glyptocrinus, in which we find associated with certain primitive 
characters a high degree of perfection. It is less perceptible among the 
Inadunata, in which, notably in the Hybocrinidx, Cystidean features are 
strongly intermingled with the characters of the larval Crinoid. But even 
among them it is impossible with the knowledge we have, or are likely to 
obtain, to form a conjecture as to the group of Cystids from which they are 
originated, and this is readily explained if we consider that the two types 
followed independent lines of development, and departed from one another 
more and more in geological time. 
The general tendency of the Crinoid type, taken as a whole, has been 
toward pentamerous symmetry, and in this they differ essentially from most 
of the Cystids. But the pentamerous tendency had to struggle with other 
tendencies, which in various ways from time to time carried one or the other 
of the subordinate groups far off in other directions. A disturbance of this 
kind was caused by the introduction of anal plates, by means of which the 
pentamerous symmetry was temporarily disturbed by a bilateral one, which 
for a time threatened to overshadow the former, until finally after the elimi- 
nation of those plates the pentamerous symmetry was permanently restored. 
The phases through which the anal plates pass in geological time in the 
various groups are well represented by individual growth in the larva of 
Antedon, and have proved to be excellent characters for family and generic 
divisions. 
The earliest fossil Crinoids have no special anal plate, and were more or 
less strictly pentamerous. Among the Lower Silurian Camerata the anal 
x is represented only in the Reteocrinidx and in the abberrant genus Comp- 
socrinus ; in all others the plate is wanting. It is absent also in the genus 
Ichthyocrinus, one of the earliest forms of the Articulata, and, as we think, 
the precursor of a large series of genera with anal plates. It probably 
was represented earlier in the Fistulata than in the other groups, as might 
be expected, for among them all tendencies toward further development 
seem to have been exhibited upon the posterior side. 
But there were other influences, not due to the anal plates, and not di- 
rectly traceable to anything shown by embryology, which not only disturbed 
