178 PROCEEDINGS OF THE ACADEMY OF [1881. 



which his Ichthyocrinidae and Taxocrinidse substantially agree 

 with our Ichthyocrinidae, except that he included among the Taxo- 

 crinidii^ the genera Lecythocrinus and Gissocrinus^ wliicli we 

 refer to the Cyathocrinidai. Our Cyathocrinidae include his 

 Poteriocrinidae, Heterocrinida?, Cyathocrinidae and Hybocrinidae, 

 except that he places among the first of these families, Agassizo- 

 crinus and Belemnocrinus^vf\\\c\i we think belong to other families. 



Zittel's classification, in its general results and conclusions, 

 does not differ materially from our own ; but instead of subdividing 

 the Palaeocrinoidea at once into a number of small groups which 

 he calls families, we separate them at first into comparatively few 

 well-marked groups, which we subdivide when necessar3^ Our 

 families are not based upon mere differences in the arrangement 

 of the plates, but are expressions of important modifications in 

 the structure of the animal, which must have affected the whole 

 organism, and consequently form the basis of well defined natural 

 divisions. 



The groups which we recognize as Ichthyocrinidae, Cyatho- 

 crinidae and Sphaeroidocrinidae existed at the beginning of the 

 geological record, and flourished side by side until they became 

 extinct. They are so well defined by nature, that once under- 

 stood there is no difficulty in identifjang them. The smaller 

 groups into which we have divided the Sphaeroidocrinidae, are 

 likewise of early origin, but they follow more or less the same 

 general plan in the arrangement of their plates, as well as in their 

 mode of development, individually and paleontologically. 



There haA^e been several interesting publications on Crinoids 

 during the past 3'ear, containing, among others, descriptions of 

 new species of both Ichthyocrinidae and Cj^athoerinidae. These 

 species will be noticed and systematically arranged in an appendix 

 at the end of this work. 



We are under special obligations to Prof. Spencer F. Baird for 

 access to a number of rare and valuable books in the Smithsonian 

 Library; to Dr. C. A. White, of the National Museum, for 

 numerous favors received during the preparation of this work ; to 

 Prof. A. H. Worthen, of Springfield, 111., and to a number of other 

 gentlemen for the liberal loan of books and specimens, and for 

 other valuable information. 



