C \l.\Moc|;iM s DIOMED 1 M 



The faintness of the lines separating the .- 1 j > i < - .- 1 1 plates of Tiareohinus 

 referred to by Love'n in his examinations of the original specimen described 

 bj Neumayer, is characteristic of all young Echini, and is well marked 

 in the young of the Arbaciadae. See mj figures, and those of Garman 

 and Colton. 



Dr. Duncan,* fully recognizing the important features in the structure 

 of the interambulacral areas, has established a new order for this genus, 

 the Plesiocidaroida, which he places among the Palaeechinoidea. This 

 had also boon suggested by Neuinayer.f 



Duncan in his Revision of the Echinoidea has also adopted for Echini 

 the nomenclature proposed by Love'n for Crinoids. The adoption of a 

 crinoidal nomenclature to designate tho apical plates of the Starfishes, 

 Sea-urchins, and Ophiurans seems to me open to verj serious objec- 

 tions. While it is true that homologies may bo more clearly indicated, 

 yet we thus lose sight of the specialization which has taken place in 

 each typo, and the direction which the development of the orders has 

 taken in palseontological time. We make loss confusion for our suc- 

 cessors by retaining within each order the special designations for the 

 different plates, and homologizing to our heart's content; but in the pres- 

 ent stage of the discussion, to adapt the nomenclature of the Polmatozoa 

 to tho orders which are contrasted to them by the very adoption of the 

 name Polmatozoa adds nothing to the accuracy of our notions of tho struc- 

 ture of the Eehinoderms. We are taking it for granted that the Starfishes. 

 Sea-urchins, and Ophiurans are the direct descendants of the Crinoids. — 

 a proposition which, from our present knowledge of the fossil types, is 

 only guesswork. We know as yet too little, not only of the homologies, 

 but also of the structure, of the Cystideans to enable us to trace their 

 development into either of the groups composing the Echinozoa, although 

 they show in many directions affinities with the other groups of Eehino- 

 derms ; and very ingenious hypotheses have been made | to show how it 

 would be possible for such forms as Mesites, found in the Lower Silurian 

 of Russia, to pass on the one hand to the Asteriadaa, and on the other 

 to the Echinidse. 



The study of fossil Ophiurans would, it seems to me, throw a good deal 



* P. Martiu Duncan, A Revision of the Genera and Great Groups of the Echinoidea, Journal of the 

 Linnsean Society. XXIII., 1889. 



t Die Starame d. Thierreichs, 1889, p. 367. 

 % Neumaver, Morphol. Studien, p. 159. 



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