94 ■ 8. LUVEN, UN I'OUUXALEHIA, A GENUS OF ECIIINOIDEA. 



I liiivc liiiziirded liero tliis attempt to explain the origin of the .Spatangean double 

 .slei'iniin in the liope tliat other observers, more t'ortunato in possessing richer nuiterials, 

 will deem it worth a strict examination. At present, this theory may eventually lie 

 found to stand the test or not, the plate marked 5 6 ^ in the skeleton of Urecliinus 

 Naresianus Al. Ag. is to be set (hjwii as homologous to the b b 2 in that of Ofl'a- 

 ster, Anancitcs, Holaster, Hemipneustes, and its b a 2 homologous to their 5 a 2, 

 This striking character of the interradium 5 in the earliest among the Spatangida", 

 tlius met with again in Urechinus; its calycinal system, ethmophract and lengthened as 

 in CoUyrites and the Holastridas; the absence of any heteronomy in 1, and the com- 

 plete symmetry of the interradia 1 and 4, recalling CoUyrites and apparently Hemi- 

 pneustes; the coalescence of a 2 and b 2 known hitherto in the certainly very old- 

 fasliioned Prymnadete, Pahi?ostoma, alone; the similarity of the five ambulacra, all of 

 them level with the general surface; the subcircular form of the peristome; the narrow 

 adoral margin of the labrum not expanded transversely, and protruding as in. the higher 

 iSpatangidie, — all these features combine to set forth the genus Urechinus — along 

 with Cystechinus and Calymne — as a true living member of the group of the Meri- 

 dosterni, by whicli the Spatangean type was tirst introduced, in tlie seas of the Meso- 

 zoic period, and A\hich was long believed to be extinct. If Urechinus Naresianus — 

 or, lor wliat we know, a Cystechinus or a Calymne — liad been found fossil in some 

 .Secondary or Tertiary stratum, any zoologist would have referred it, without hesita- 

 tion and rightly, to the »Ananchytida))); — but at the same time one feature would 

 have caught his attention as strikingly peculiar and distinctive, the total absence in 

 tiie ambulacra of any trace of petaloid structure. 



The recent Spatangidte that live in the littoral belt, and the allied fossil forms 

 of Prymnadetes and Prymnodesmians, all have, for a common character, the dorsal 

 portions of their paired ainl)ulacra, II and IV, I and V, transformed into more or less 

 developed petala, within whicli the plates are crowded, shortened while transversely 

 extended, and freipiently mure or less deeply sunk beneath tiie perisome, all this in 

 order to afford as large a space as possible to the increased number of |)edicels changed 

 into triangular, compressed leaflets, evidently subservient to respiration. Now, it will 

 have been remarked that in certain generic forms of Prymnadetes and Prymnodesmians 

 found to inhabit the gTeat depths, the petals, when compared with those of the properly 

 littoral forms, are seen to be but feebly or not at all developed. In Homolampas 

 their plates arc but sligl'tly shortened, and tiie minute perforations of their pores are 

 placed diagonally, not transversely; ■ — and Argopatagus and Paheotropus are entirely 

 apetalous. The littoral forms of tlie Schizasters are provided with highly developed, 

 deeply sunk petala, while their representatives in the great depths, Aceste and Aerope, 

 in this point absolutely contrast with them, having the paired ambulacra wholly apeta- 

 lous, narrowed in their dorsal portions, and level witli the perisome. Thus, while in 

 tlie littoral Spatangidie a tendency universally prevails towards having their dorsal 

 ambulacra with their ])cdicels modihed for branchial functions, there seems to obtain, 

 in the abyssal forms, within this part of their vital economy, a quiescence, that leaves 

 these same ambulacral plates and pedicels in undifferentiated simplicity, — a peculiarity, 



