SYSTEMATIC PAET. 147 



Silurian and perhaps Cambrian age, is indicated bj 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 Hybocrinid^, 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 Reteocrinidag 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 



