EEVISION OF PALEOZOIC STELLEROIDEA. 11 



struction of the skeleton as it was in life, will learn where to separate 

 and what to reject. 



During the past 10 years, Schondorf has revised most of the German 

 asterids and has studied deeply into the structure of the ophiurids. 

 His work is very detailed and the best extant on the Paleozoic forms. 

 Every student of Stelleroidea intending to do thorough work will 

 have to examine his results. In classification he has laid the founda- 

 tion, and has shown that some asterids are not Asteroidea at all, but 

 that they and the bulk of the so-called ophiurids of the Paleozoic must, 

 because of their peculiar structure, ambulacral and otherwise, be 

 referred to another class, the Auluroidea. 



In regard to the evolution of the Asteroidea, the writer hoped 

 to find the time to present his views concerning it in detail in this 

 memoir, but that also is not possible. However, the main lines 

 and often the generic directions of organic change are stated and 

 will be found either in the introductory pages or scattered through- 

 out the work, generally in the remarks under the generic discussions. 

 These results in connection with the work of Schondorf, it is thought, 

 ought soon to place the Paleozoic Stelleroidea in such order that 

 a final classification, based on ontogeny, chronogenesis, and phyl- 

 ogeny, and embracing not only the Paleozoic forms but those of 

 Mesozoic, Cenozoic, and Recent time as well, can be made. 



Many museums and individual paleontologists have loaned the 

 writer the specimens in their collections or in their keeping, and he 

 wishes here to thank them, one and all, for these loans. The greatest 

 bulk of the material studied is of course in the United States National 

 Museum in the Harris and Ulrich collections; a number of Trenton 

 specimens and some from the English Siluric have also been given 

 to the National Museum by Mr. Walter R. Billings, of Ottawa. 

 Since the writer has been in New Haven, Dr. R. S. Bassler, of the 

 National Museum, has often kindly helped him to further his studies, 

 and during the past year has made a large number of photographs 

 of the specimens under his ' charge. The many Mississippic speci- 

 mens in the Frank Springer collection have not been studied. 



The author is greatly indebted to Dr. F. A. Bather, of the British 

 Museum (Natural History), for a large number of gutta-percha and 

 wax squeezes made by him of certain species in that great museum. 

 These casts, which are all in the United States National Museum, 

 have enabled the writer to understand several genera that otherwise 

 he could not have worked out. He is also indebted to him for many 

 bibliographic corrections and suggestions. 



The second largest American collection is at Harvard University, 

 in the Museum of Comparative Zoology. Most of the material is 

 in the Charles D. Walcott, Charles B. Dyer, and Charles Wachsmuth 

 collections. In the first-named collection there is excellent material 

 from the Middle Ordovicic; the second has many fine specimens 



