KONGL. SV. VET. AKADEMIENS PIANDL. BAND 19. N:0 7. 71 



and the radials I and V, in the old subglobose forms with a normally construcled ster- 

 num: Epiaster, Isaster, and the typieal species of the abundantly developed Hemiaster, 

 Pl. XVIII, fig. 221, 222, while in other species'), from the middle Cretaceous era, it 

 is closed also, but solely by the radials I and V, the costals 1 and 4 having separat- 

 ed, in this respect like the calyx of Micraster"), the first of Piymnodesmians, contin- 

 uing from the middle Cretaceous into the Tertiary period. In all these forms the 

 madreporite is strictly confined to the right anterior costal, 2. They may be conveni- 

 ently comprised under the common name of Ethmophracti^). But, of them all, as far 

 as our knowledge extends, no generic type has survived up to the present time save 

 Hemiaster, while the abyssal depths contain some genera of Ethmophracts, as Calymne, 

 Urechinus and Cystechinus, that more or less recall those ancient forms. 



For in the latter half of the Cretaceous era an important change took place 

 in the structure of the calycinal system of the Spatangida?. It has been seen that, 

 when, in the Echinoconidse, the periproct had retreated far back from the calyx, the 

 costal 5, which had been suppressed, was reinstated again, and that the normal con- 

 dition returned even so far as to allow the efferent duct of the corresponding sexual 

 gland to perforate it. In a similar manner also among the Spatangida?, the central 

 disk and the costal 5 make their reappearance, the former separating the costals 1 and 

 4, the latter disjoining the radials I and V, but never receiving a sexual pore. At the 

 same time the madreporic filter, no more held back by the excretor}' apparatus, but 

 free to move and expand, spreads its pores, as formerly in the Cassidulidaj, into the 

 now extended space of the central disk, and from thence into the costal 5. The Spa- 

 tangidfe provided with a calyx of this description may be distinguished as Ethmoly- 

 sii*). The madreporite, while it penetrates the substance of the costal 2, the central 

 ossicle and the costal 5, obliterates the sutures which separated them, as it did the 

 suture between the 2 and the central in the Echinoconidse, Echinoneida! and Cassidu- 

 lidae, and those three ossicles, so strictly kept apart in the Endocyclic forms, are here 

 united into one unbroken area, contiguous anteriorly to the interradium 2, and poste- 

 riorly to the interradium 5. It will appear that Linthia and Schizaster") from the middle 

 Cretaceous time to our time, Prenaster from the end of the Cretaceous period and far into 

 the Eocene, Macropneustes in the Eocene, were the first in which this change took place, and 

 associated with them may perhaps be found some species rightly beloiiging to Abatus''), 

 Pl. XVIII. jlg. 220. In thr-ee of these oldest genera of the Spatangida3 Ethmolysii: 

 Schizaster'), Prenaster, Macropneustes, the costal 5 is not extended far beyond the poste- 

 rior limits of the s}'^stem, and in Schizaster the madreporite takes its due by more or 

 less crowding with its pores the costal 2, so as to cause the abortion of the efferent 



') Mainly, or wholly!', algerian. See Cotteau, Peron et (jauthikr, Éoliinides fossilcs de rAlgérie. IV, 



Cenomaiiieii, VI: Tiironien. 



2) Études, Pl. XI, fig. 95. 



•*) 'Fld/.tf'iQ, filter; (pQay.znc, fenced. 



*) ylvQlOiz, deliverer. 



^) Schizaster iintiquus Cotteau, BuU. Soc. Géol. VI, 567. 



") Corapare Abatus Pliilippii, Études, pl. XI, tig. 99. 



') Schizaster fragilis, Ib., pl. XII. fig. 102. 



