ON THE STEUCIUEE OF THE AMEULACKA OF ECHINOIDEA. 419 



31, On the Steticttjee of the Ambtjlacea of some Possil Gteneea and 

 Species of E/Egulae Echiijoidea. By Prof. P. Maetin Duncan, 

 M.B. Lond., P.R.S., P.G.S. (Pead April 29, 1885.) 



Contents. 



I. Introductory Remarks. — II. General Remarks on the Structure of Ambu- 

 lacra. — III. Descriptions of the Ambulacral Plates of some Genera and Species, 

 and the necessity for the Introduction of Diplopodia, McCoy, and a new Genus, 

 Pledodiadema. — IV. Conclusions relating to the types of Ambulacra. — V. De- 

 scription of the Figures. 



I. Inteoductoey Pemaeks. 



The characters of the ambulacra of the Echinoidea have always been 

 considered of primary importance in the classification of that great 

 division of the Echinodermata. 



The details of the structures of the ambulacra of the regular 

 Echinoidea have been long known in the instance of the Cidaridge, 

 in some of the Salenidce and Echinometridse, and in many genera of 

 Echinidae. 



The study of the structures led to the separation of the groups of 

 genera with simple plates and one pair of pores to a plate from 

 those having compound plates with three or more pairs of pores- 

 Cidaris has been acknowledged as the type of the first group, and, 

 thanks to the elaborate investigations of Loven *, Strongylocentrotus 

 may be considered the type of the other series. 



In a monograph Dn the fossil Echinoidea of Sind, by the 

 author and Mr. W. Percy Sladen, E.G.S., there is a description 

 of two species of Ccelopleurus from the Oligocene, and the illus- 

 trations which accompany the work fully explain that there is 

 a type of ambulacral plate which departs from the received idea 

 regarding the intimate structure of plates with triple pairs of pores f. 

 Subsequently some researches by the same authors proved that the 

 Arbaciadse and the recent Diadematidae had their ambulacra con- 

 structed on a plan hitherto unobserved, and which separated the 

 groups distinctly +. Having obtained a knowledge of the characters 

 of the recent forms, the study of the corresponding details of the 

 fossil species became tolerably easy. 



The results of this study are now offered to the Society. 



I have to thank the executors of the late Dr. T. Wright, P.P.S., 

 for permitting me to examine and draw what was necessary from 

 the beautiful specimens in their charge. I am also very glad to 

 have the opportunity of thanking the authorities at the British 

 Museum and at the Museum in Jermyn Street for allowing me to 

 study the collections. 



In the majority of fossil regular Echinoidea the pairs of pores of 



* Loven, Etudes sur les Echinoidees, 1874, p. 19. 



t ' Palasontologia Indica,' ser. xiv. fasc. iv. pi. xxxix. (1884). 



I Journ. Linn. Soc, Zool. vol. xix. pp. 25 and 95 (1885). 



