KONGL. SV. VET. AKAOEMIENS HAN'DL. BAND 19. N:() 7. 71 



and the radials I and V, in the old subglobose t'oi-ni.s with a normally constructed ster- 

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

 Fl. XVIll, jii/. 221, 222, while in othei' species'), from the middle Cretaceous era, it 

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

 ed, in this respect like the calyx of Micraster'), the first of Prymnodesinians, 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. Thev 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 tlie Cretaceous ei'a an important change took place 

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

 when, in the Echinoconidie, the periproct had i-etreated far back from the calyx, the 

 costal 5, which had l)een 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 simihir manner also among the Spatangidaj, the centi'al 

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

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

 same time the madrepori(- filter, no uku'c held back by the excretory apparatus, but 

 free to move and expand, spreads its pores, as fornicrlv in the Cassidulid», into the 

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

 tangida^ provided witli a caly.x of this description may be distinguished as Ethmolv- 

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

 ossicle and the costal .3, t)bliterates the sutures which separated them, as it did the 

 suture Ijetween the 2 and the central in the Echinoconida'. Echinoneida; and Cassidu- 

 lidaj, and those three ossicles, so strictly kept a])art in the Endocvclic forms, are here 

 united into one unbroken aj'ca, contiguous anteriorly to the interradium 2, and poste- 

 riorly to the interradium 5. It will apjiear 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 belonging to Abatus*"), 

 PL XVJII, Jig. 220. In three of these oldest genera of the Spatangidaj Ethmolysii: 

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

 rior limits of the system, 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 Ootteau, Pkron r\ (Jm iiiiiit, (■Icliiiiiik-s fossiks de IWlgerie. IV, 



Cenomaiiieii, VI: Turonien. 



■-) Etudes, PI. XI, fig. 95. 



^) ' HSfioc, ftlter; cpQnxT('ic, fenced. 



*) AiKiOQ, deliverer. 



'^) Schizaster iintiquus Cottkau, Bull. Soe. Ueol. VI, 507. 



") Compare Abatus Pliilippii, Ktudes, pi. .XI, lig. 'J9 



'') Schizaster t'ragilis, lb., pi. .\ll, fig. 102. 



