KONGL. SV. VET. AKADEMIENS IIANDL. BAND. 19. N;0 7. 79 



calls certain ancient types, and at the same time is so like its contemporaries in the 

 structure of the peristorne, the sternmn, and the episternum traversed by the subanal 

 fasciola, as also in the regular heteronomy of 1, a, presents a calycinal system, Pl. 

 XVII, jig. 208, of a nearly pentagonal outline, iu Avhich the radials are distinct, the 

 I and V being widely separated, while the costals are all coalesced into one piece 

 devoid of sutures, and in which the madreporite opens anteriorly in the middle by 

 a small lissure and some pores, while the genital ducts are only two, having their 

 outlets at the tops of tAvo large tubular eminences placed transversely against the in- 

 terradia 1 and 4. 



Palffiostoma mirabile Gray, PL XVI, deviates in a stränge manner from nearly all 

 the rest of the Spatangidse '), by the fusion into one singie plate of the second plates 

 of- the interradia 2, 3, 4, the heteronomy of 1 being effected through the union of the 

 plates a 2, b 2 and b 3; by the very irregular interradium 5, and by the pentagonal 

 peristorne Avith its tive valves, and from Palteotropus in particular, by its distinct pet- 

 als and by the absence of a subanal, the presence of a peripetalous fasciola. But 

 Avith all this, Palseostoma offers a calycinal system evidently constructed upon the same 

 model as in that genus, jig. 184, 190, vJiXh the fiA^e radials distinct, the I and V Avidely 

 separated, and out of the costals the 3 alone defined by a suture, all the rest being 

 coalesced into one piece; Avith the madreporic filter represented, in the young speci- 

 niens examined, by a fcAv punctures placed before the middle, and Avith the two huge 

 sexual outlets, mammiform and prominent, occupying a considei'able portion of the 

 system, and placed transversely against the interradials 1 and 4, so as to prevent the 

 retrograde passage of the madreporite. These tAvo genera, Palasotropus and Palajostoma, 

 are, therefoi'e, in a general sense Ethmophracti, but after a peculiar manner, entirely 

 different from that Avhich characterises the older Spatangidse comprised under this 

 appellation. They may be called Perissogonea ^). There is not in any other genus of the 

 Echinoidea, recent and fossil, anything strictly comparable to this structure. In some 

 points it recalls Avhat is seen among the Exocyclic Gnathostomes and the earlier Ate- 

 leostomes. The radials I and V are kept asunder by the interposition of the costal 

 5, and the madreporite spreads its pores över the entire central area, Avithout any 

 tendency to move backwards or out of the calycinal system. Thus it is in Holectypus 

 and Discoidea ^), of Oolitic origin, in most of the Cretaceous Echinoconi, in the re- 

 cent Cassidulida3 ''), in the Clypeastridaä "), of Tertiary origin. In these tAvo last the su- 

 tures have a tendency to be obliterated. But in all of them the sexual pores are five 

 or four in number, and placed against their respective interradia, into Avhich they not 

 seldom are transferred. 



1) Études, p. 12, 50, Pl. XII, lig. 103, 104; XXXII, fiir. 197 — 199. — A fossil species of tliis genus, 

 from the Nummulitic strata of Western Sind, has been described as Hemiaster elongatus Duncan and 

 Släden, Mem. Geol. Snrv. India, Tert. etc. Ser. XIV; Vol. I, 3, p. 78, pl. XIX, fig. 7 — 15. Calcutta 1882. 



^) UsQlsgös, exoessive, l'ovrj, generative organ. 



3) Études, p. 81, pl. XV, fig. 133, 132. 



') Ib., fig. 130. 



5) Ib., pl. XVI, fig. 135—139. 



