THE FAMILY AEBACIAD^. 27 



of Arhacia in the National Collection, with the assistance of 

 some specimens belongiug to us ; and it has resulted that we can 

 offer to those students of the Echinoidea who are interested in 

 this family, some descriptions of new morphological details which 

 must affect the classification. 



The communication which we bring before the Society consists 

 of two parts. In the first, the nature of the structures of the 

 ambulacra, of the interradial plates, and of the radial plates in 

 the fossil and recent Coelopleuri and in the species of Arhacia 

 are considered. In the second part we shall consider the 

 classification of the family. 



We use the name Ariacia because Gray, in our opinion, has 

 the precedence over Desmoulins, whose name ^chinocidaris is, 

 however, much the better of the two. 



Grenus C(elopletjeus, L. Agassiz. 

 II. The Fossil Species of CcBlo^XevLVM^, L.Ag.^ 



The Ambulacra. — The species which were examined were 

 C equis, Ag., G. Pratti, d'Archiac, C. Forhesi, d'Archiac, and 

 G. sindensis, nobis t. 



The ambulacra of the species of Goelopleurus which are f ouud 

 fossil may, for the purpose of description, be divided into three 

 regions — the region of the ambitus, that of the peristome, and 

 the apical region. The arrangement of the plates differs in each 

 of these divisions ; but it is evident that in the fossil forms, as in 

 the recent, the plates of the aboral region were the last formed, 

 the peristomial are the oldest, and the plates of the ambitus are 

 of an intermediate age. It is also evident that, in common with 

 other Echinoidea with multiple or combined plates, the Goelo- 

 pleuri present the simplest plates in the aboral region of the 

 ambulacra and the most complicated in the peristomial portions, 

 but the complication is very slight there in the genus now under 

 consideration. 



All the fossil species of the genus Gcelopleurus from the Oligo- 



* The literature of the Arbaciad^ is carefully given in the ' Revision of the 

 Echini ' by Alex. Agassiz, and we may add two references of work which relates 

 to that we now describe : — Loven, ' Etudes sur les Echinoidees,' Stockholm, 1874 ; 

 Troschel, Archiv fiir Naturgeschichte, Wiegmann, 1872, p. 293, and 1873, p. 308. 



t " Fossil Echinoidea of Sind:" Nari and Graj Series, 1884^1885 ; "Fossil Echi- 

 noidea of Kachh and Kattywar," 1883 : see 'Palseontologia Indica,' Series XIV. 

 (Duncan and Sladen). 



