172 PEDINA. 



to prove that the bosses are smooth, and without any trace of crenulation, in this genus ;* 

 the accuracy of this observation has been confirmed by M. Cotteau,t who has added 

 the following note to his description of Pedina aspera .• " M. Wright est le premier qui a 

 constate contrairement a 1' opinion de M. Agassiz, que les tubercles des Pedines sont cer- 

 tainement depourvus de crenelures. Sur tous les echantillons de Pedine, que nous avons 

 examines depuis, nous avons ete a meme de reconnaitre 1' exactitude de cette observation." 



When I first pointed out the true structure of the bosses in the Pedinas to my late 

 colleague, Professor Edward Forbes, he considered the fact of so much importance, that he 

 proposed to suppress the genus Pedina, and merge its species into Echinopsis, as one of 

 the characters given of EcJdnopsis was that " the tubercles were perforated but not 

 crenulated." For this reason, in the class Echinodermata in Morris's ' Catalogue of British 

 Fossils,' from the pen of Professor Forbes, and likewise in lettering Plate 8, Decade V, of 

 the ' Memoirs of the Geological Survey,' this view was carried out. 



The genus Pedina was well described and figured by Agassiz in his ' Echinodermes 

 Fossiles de la Suisse,' and its limits accurately indicated, before Echinopsis was proposed. 

 Assuming, therefore, that the diagnosis of both genera was equally well defined, which is 

 not the case, still Pedina has the priority. 



In the absence of good type specimens, it is impossible to make a correct comparison 

 between Echinopsis and Pedina from the definitions in the books ; and if M. Desor, who 

 has lately carefully examined the types of Echinopsis, declares the genus to be unsatis- 

 factory, this is another reason why we should retain Pedina. In his note on 

 this genus, M. Desor j says : " Mais meme tel qu'il est ici defini, le genre Echinopsis est 

 encore moins precis qu'on ne pourrait le desirer, car il renferme des especes a pores 

 simples et d'autres a pores dedoubles. Mais comme ces deux types sont si voisins sous 

 tous les autres rapports, particulierement par leur forme renflee, la petitesse de leur peri- 

 stome et la structure de leur plaques, je n'ai pas cru devoir les separer generiquement. 

 Je me suis borne a en faire deux groupes." 



The genera Pedina and Echinopsis are the only ones in which we find perforated 

 tubercles combined with trigeminal pores ; but in Echinopsis the ambulacral areas are 

 nearly as wide as the inter-ambulacral, which is not the case in Pedina. 



The Pedinas have perforated tubercles, with smooth bosses, combined with trigeminal 

 pores, by which they are distinguished from Pseudodiadema. They closely resemble 

 Hemipedina, from which they are chiefly distinguished by the smallness of their tubercles, 

 and the trigeminal arrangement of their pores, those in Hemipedina being unigeminal ; 

 the apical disc is likewise much larger in Hemipedina. 



The perforation of the tubercles distinguishes Pedina from the true EcHiNiDiE, which it 

 otherwise resembles in having a thin test, with trigeminal pores. 



* 'Annals and Magazine of Natural History,' 2d series, vol. xiii, p. 173. 

 t • Etudes sur les Ecliinides Fossiles,' pp. 313, 314. 

 X 'Synopsis Ecliinides Fossiles,' p. 99. 



