94 N. LOVEN, UN ruUUTALIiSlA, A GENUS OK KCUIINOIJJKA. 



I)le 



1 liiivc luizardcd liere this attciiipt to cxplixiii tlic ori^iii oF tlie Spatangcun doLil) 

 stcrmiiii in the hopc tliat otlier observers, uiore fortunatcj in pussessing riclicr materials, 

 will dccni it wortli a strict cxauiiiiation. At present, this theory may cventually be 

 found to stånd the test or not, the plate marked b b 2 in the skeleton of Ureehiniis 

 Naresianus Al. Ag. is to be set down as homologous to the b b 2 \x\ that of Offa- 

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

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

 thus met with again in Urechinus; its calycinal system, etlimophract and lengthened as 

 in CoUyrites and the Holastrida;; the abscnce of any heteronomy in 1, and the com- 

 plete symiuetry 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- 

 fashioned Pryninadete, Palaiostoma, alone; the similarity of the tive aaibulacra, all of 

 theui 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 highcr 

 Spatangidtu, — all these features combine to set fortli the genus Urechinus — along 

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

 dosterni, by which the Spatangean type was first introduced, in the seas of the Meso- 

 zoic period, and which was long believed to be extinct. If Urechinus Naresianus — 

 or, lor vvliat we know, a Cystechinus or a Calymne — had been found fossil in some 

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

 tion and rightly, to the »Ananchytidaj»; — but at the same time one featurc would 

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

 the ambulacra of any trace of petaloid structure. 



The recent Spatangidaä tliat 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 ambulacra, II and IV, I and V, transformed into more or less 

 developed petala, within which the plates are crow^ded, shortened while transversely 

 extended, and frequently more or less deeply sunk beneath the perisome, all this in 

 order to afford as large a space as possible to tlie increased number of pedicels changed 

 into triangulär, 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 great 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 are but slightly shortened, and the minute perforations of their p öres are 

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

 apetalous. The littoral forms of the 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 theni, having the paired ambulacra wholly apeta- 

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

 the littoral Spatangidaj a tendency universally prevails towards having their dorsal 

 ambulacra with their pedicels moditied 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, 



