No. 7. — The Cidaridae. By Hubert Lyman Clark. 



Introduction. 



The opening years of the present century have witnessed the publica- 

 tion of an unusual number of quarto volumes dealing with the morphol- 

 ogy and classification of the Echini. In each of these the Cidaridae 

 receive considerable attention, and many genera of that family, new 

 either in name or in contents, are proposed. As the different writers 

 reveal wide divergence of opinion as to the relative importance of the 

 characters on which the classification of the Echini is based, the arrange- 

 ment of the Cidaridae differs to an unusual degree in these several re- 

 ports. Mortensen (:0s) 1 practically rejects previous classifications and 

 the principles on which they are based, and, ignoring the fossil forms, 

 to which his method is not applicable, recognizes thirteen genera and a 

 subgenus, defined wholly in terms of the pedicellariae, the spicules of the 

 pedicels, and occasionally the spines. It is only fair to state, however, 

 that the writer says frankly, these features are not " sufficient for definitive 

 diagnoses." He includes in his classification 42 species, and lists 12 

 others which he is unable to place satisfactorily because of lack of infor- 

 mation about the pedicellariae. Very soon after this volume appeared, 

 de Meijere's (:04) 2 valuable report on the "Siboga" Echini was pub- 

 lished. Unwilling to accept Mortensen's genera unreservedly, the writer 

 adopts the clumsy and unsatisfactory method of recognizing only a single 

 genus, Cidaris, and using Mortensen's names for subgenera. Later in 

 the same year Agassiz (:04) 8 in his report on the Panamic deep-sea 

 Echini, points out the weaknesses of Mortensen's method and the unsatis- 

 factory nature of his results, and emphasizes anew the great morpholog- 

 ical significance of the test (including the abactinal system). Two years 



1 The Danish Ingolf-Expedition, 4, 1. Echinoidea. Part 1. Th. Mortensen. 

 Translated by Torben Lundbeck. 193 pp., 21 pis. Copenhagen, 1903. 



2 Die Echinoidea der Siboga-Expedition. J. C. H. de Meijere. 252 pp., 23 pis. 

 Leiden, 1904. 



3 The Panamic Deep Sea Echini. Alexander Agassiz. Mem. Mus. Comp. 

 Zool., 31, 243 pp., 112 pis. 1904. 



