Wachsmuth and Springer—Silurian Crinoids. 257 
admitted and repeatedly asserted. We do not know of a single 
genus, illustrated by any considerable number of specimens, 
in which there are not some forms exhibiting a variation 
of one or more of the typical characters, and constituting 
transitional forms between that and some allied genus. The 
same may be said of some of the best defined species, and in- 
deed, this observation may be extended to almost every group, 
whatever be its rank, which naturalists have attempted to sep- 
arate and define. For example, one of the best characters of 
Actinocrinus is the simplicity of the arms, which almost univer- 
sally remain undivided after becoming free. Yet we i 
one of the latest species, A. Lowez, which otherwise retains and 
exaggerates all the characteristic features of the genus, that 
number. 
No method of systematic classification has as yet been 
devised to adequately provide for all such cases; and we do 
not believe it possible so to limit and define the characters of 
a genus, as to escape the difficulties arising from modifica- 
tions due to individual growth, and the variations of types in 
geological time. re 
A modification of the phraseology of our generic descriptions 
of Glyptocrinus and Reteocrinus, by which the existence of cer- 
tain of the less important characters, such as surface ornamen- 
tation, the number of arms, and the geometric form of plates 
should be stated in less absolute terms, and with greater al- 
lowance for exceptional cases, would apparently meet the objec- 
tions urged by Mr. Miller on this point; and we may find it 
advisable to make such alterations hereafter in this and perhaps 
other cases. It did not occur to us that our language was 
liable to misconception in this respect, in view of our frequent 
expressions as to the value of such characters, and our explicit 
Statement of the leading characters. e do not regard the 
—Turrp Serres, VoL. XXV, No. 148.—Aprin, 1883. 
