1879.] NATURAL SCIENCES OF PHILADELPHIA. 245 



in maturity become free, and a calcareous deposit is secreted around 

 the base, which covers and obliterates the sutures between the 

 plates. Here again we have an actual metamorphosis during the 

 life of the individual of five plates into one, and this seems to us 

 to be strongly confirmatory of the views of Agassiz and Loven. 

 We are inclined to think that the plate within the ring of under- 

 basals when it exists, as in Marsupites, represents the column 

 of Crinoids generally. 



Having thus discussed the relations and distinctions between 

 some groups of the Crinoids and the differences between the 

 Palseocrinoidea and the Stomatocrinoidea so we should like to 

 call all Crinoids which have an external mouth we think it pro- 

 per to indicate briefly the principles which we shall endeavor to 

 follow in our more detailed work. 



Classification. 



In attempting to make a sj^stematic classification of the Paleo- 

 crinoids into families and genera, we encounter the difficulties 

 which usually confront us when we undertake to ascertain and 

 define any divisions as they exist in nature. We can readily re- 

 cognize in groups of fossils certain broad characters by which it 

 seems natural and satisfactory to bring them together, and we 

 generally find in the characteristic types of the respective groups 

 an association of other characters, by which they appear sharply 

 marked ; and so long as we have to deal with typical forms in 

 isolated specimens or groups, the work is simple enough. But 

 when we begin to investigate large collections, and in a measure 

 to stud}' comparatively all the known material from specimens or 

 descriptions, we find the subject bristling with perplexing ques- 

 tions. Types are found to shade into one another, characters are 

 commingled through processes of transition, which sadly inter- 

 fere with the nice definitions we think we have worked out. How 

 to deal with such forms has always been a troublesome question 

 with naturalists, and the diverse methods of treating it have given 

 rise to much confusion. We have found it especially perplexing 

 in endeavoring to define the genera of the Crinoids. We find for 

 instance, two groups, each embracing a number of species, and we 

 discover general characters which nicel}* separate them. Further 

 researches presently reveal to us certain forms, including perhaps 



