CALAMOCRINUS DIOMED^. 81 



The faintness of the lines separating the apical plates of Tiarechinus, 

 referred to by Loven in his examinations of the original specimen described 

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

 in the young of the Arbaciadaj. See my figures, and those of Garman 

 and Col ton. 



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 Pala^echinoidea. This 

 had also been suggested by Neumayer. f 



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

 the nomenclature proposed by Loven for Crinoids. The adoption of a 

 crinoidal nomenclature to designate the apical plates of the Starfishes, 

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

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

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

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

 taken in palajontological time. We make less 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 Pelmatozoa 

 to the orders which ai'e contrasted to them by the very adoption of the 

 name Pelmatozoa adds nothing to the accuracy of our notions of the struc- 

 ture of the Echinoderms. 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 

 the}- show in many directions affinities with the other groups of Echino- 

 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 Asteriadaj, and on the other 

 to the Echinidse. 



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



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

 Linnaean Society. XXIII., 1889. 



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

 X Neumayer, Morphol. Studien, p. 159. 



