()0 S. 1,()VRN, ON I'OUUTALESJA, A GENUS OK ECHINUIDEA. 



the petala, are in tlieriiiselves uninterrupted, BopeiD) eoutinuutiotis of the ainbulneral 

 series of phitcs, thus iii Cassiduhis, in the Adetes, in Spatangus, Mieraster, Maretia, 

 Echinocardiuin, Lownia. When, in others, as in Agassizia, Meoiiia, Brissopsis, Kleinia, 

 Brissus, Eup;itagus, 1'lagionotu.s, lireynia, they are »chised» inferiorly, this is an acei- 

 deiital feature, tlie constriction being caused solely by the pressure exerted by the 

 perii)etalous fasciola. ') The petaline plates are longitudinall}' narrow, extended trans- 

 versely, and the peripodiunj, generally placed towards the external margin, incloses 

 with a narrow rim the always geininous, widely separated, transversely placed per- 

 forations, and unites them with a slender compressed ridge or with a furrow, a more 

 or less distinct suture marking the junction of the margins from either side. Upon 

 the Avhole a structure like this is found prevalent in the great majority of forms com- 

 ing into view from tiie iirst appearance of the Cassidulida?. But among the Spatan- 

 gidaj their exists a group, of ;i few genera, differing widely enough in other respects, 

 but held together by a common character, the absence of petala. They are Apetalous; 

 whether Abraiichian further researches will decide. Their ambulacra, instead of 

 being, in theii- dorsal portions, deepened and crowded with compressed plates, in order 

 to give room to numei'ous gill- leaflets, are all along even with the general surface, 

 and oidy gradually contracting up to the top, and their plates, as high as they are 

 bi'oad, or nearly so, are regularly hexagonal, the pores being very minute, placed cen- 

 trally or subcentrally, and the pedicels smiill and simple. These genera are Pala30- 

 tropus, Argopatagus; Urechinus; Cystechinus; Genicopatagus; Aceste, Calymne, Aerope. 

 They are all abyssal. 



In the vast majority of Echinoidean forms, Arhaionomous and Neonomous, — 

 and ])resuniably in the whole of them, — it is seen that the adoral and inner perforation 

 of the peripodium is prolonged into a short and narrow slit, cut through the wall, and 

 tliat this slit widens below into a separate but smaller perforation. This is very di- 

 stinctly seen in tlie Cidaridffi, Echinida3, Echinoconidix;, Echinoneidic, and, above 

 all, in the j)hyllodean peripodia of the S])atangidae. in those of the Cassidulida;, as 

 generally in the subanal and frontal, as well as petaline peripodia, it is less distinct, 

 but mostly to be recognised as a minute notch in the wall. This is the particular 

 little foramen tliat gives passage to the branch of the ambulacral nerve which is seen, 

 on the inside of the plate, to enter the pore along with the vessel, and, as easily, on 

 the (nitside, to emerge from it anrj distribute its branchlets, all tlirough the connective 

 tissue, to the external organs. ") 



Such is a sketch of the pedicels in the most prominent of Neonomous Echinoidea, 

 as they are variously specialised in order to meet reijuirenu'nts nu)re varied than those 

 essential to the earlier types: ih'veloped into delirate organs of touch, or combining 

 with tnctual function tliat of prehension, modified foi- purposes still obscure, or evi- 

 dently subservient to respiration. I have dwelt at some length on these diversities, 

 with a view not only to add a few facts more to those known already, but mainly to 



') Etudes p. 62. -} lb., p. 8, pi. II, fig. 29, 30, 31. 



