Oeological Society. 231 



structure of the ambulacra in the Cidaridse and the elaborate inves- 

 tigations of Loven on the Triplechinidse, the author brought before 

 the Society the results of his own work with and without the co- 

 operation of his fellow-worker in the description of the Echinoidea 

 of Sind, Mr. Percy Sladen, F.G.S., and which referred to the Diade- 

 matidse and the Arbaciadse of the recent faunas. Starting with the 

 knowledge of the construction of the modern Diadematidae, the 

 author investigated the genera Hemipedina, Pseudodiadema, Pedina, 

 Hemicidaris, Piplojoodia, and CypTiosoma. The necessity for the 

 reestablishment of the genus Diplopodia was shown, and a new genus, 

 Plesiodiadema, was founded. Pseudodiadema, shorn of the forms 

 included in these genera, remains, and differs more from Diadema 

 than has been believed. The method of the growth of the great 

 plates of Hemicidaris was explained, and the comparison between the 

 peristomial plates of some of the Diadematidae and the universal 

 structure of the ambulacral plates in Pedina was made. The author 

 considered that there are six types of ambulacra in the regular 

 Echinoidea, so far as the group has been investigated, there still 

 remaining much to be done. These types are the Cidaroid, Diadema- 

 toid, Arbacioid, Echinoid, Cyphosomoid, and Diplopodous. In con- 

 clusion the succession in time of the structures which characterize 

 these types "was considered. 



May 13, 1885.— Prof. T. G. Bonney, D.Sc, LL.D., F.E.S., 

 President, in the Chair. 



The following communication was read : — 



" On the Ostracoda of the Parbeck Eormation ; with Notes 

 on the Wealden Species." By Prof. T. Eupert Jones, E.R.S., 

 F.G.S. 



The author stated that in 1850 Prof. Edward Forbes had determined 

 the tripartite division of the Purbeck beds, after working at the sec- 

 tions in the south of England with Mr. Bristow, and had intimated 

 that several species of the so-caUed " Cypridae " aided him in arriving 

 at this result. He did not, however, publish any account of the several 

 forms, and we know of his intended species only (1) by his having 

 pointed them out to his friends Messrs. Bristow, Osmond Fisher, and W. 

 Cunnington ; (2) by a letter to Mr. Bristow in 1851 and one to 

 the Author in 1854; (3) by some diagrams in the Museum of 

 Practical Geology ; and (4) by some rough woodcuts in Sir Charles 

 LyeU's 'Manual of Elementary Geology,' 5th edit. (1855). Having 

 a large collection of Purbeck and Wealden Entomostraca, the author 

 has endeavoured to decide which were E. Forbes's species ; and 

 from a careful examination of the collections in the Geological 

 Society's Museum, the Museum of Practical Geology, and the 

 British Museum, in which he has been greatly assisted by Mr. E. T. 

 Newton, F.G.S., and Mr. C. D. Sherborn, he has arrived at the 

 definite conclusion that there are fourteen species in E. Forbes's 

 three divisions of the Purbeck series. Five of them {Cypris pur- 

 heckensis, Gccndona bononiensis, 0. ansata, Gythere Bla/cei, and 0. 



