KONGL. SV. VET. AKADEMIENS HANDL. BAND. 19. N.O 7. 79 



rails certiiiu ancient types, and at the same time is so like its contemporaries in the 

 structure of the peristome, the sternum, and the cpisternum traversed by the subanal 

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

 XVII, fig. 208, of a nearl^^ pentagonal outline, in which 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 fissure and some pores, while the genital ducts are only tAvo, having their 

 outlets at the tops of two large tubidar eminences placed transversely against the in- 

 terradia 1 and 4. 



Palajostoraa mirabile Gray, PL XVJ, deviates in a strange manner from nearly all 

 the rest of the Spatangidfe '), by the fusion into one single 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, I) 2 and I) 3; by the very irregular interradium 5, ajid by the pentagonal 

 peristome with its five valves, and from Palaeotropus in particular, by its distinct pet- 

 als and b}' the absence of a subanal, the presence of a peripetalous fasciola. But 

 with all this, Pala^ostoma offers a calycinal system evidently constrticted upon the same 

 model as in that genus, fig. 184, 190, with the five radials distinct, the I and V widely 

 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- 

 mens examined, by a few punctures placed before the middle, and with the two huge 

 sexual outlets, mammiform and prominent, occupying a considerable portion of the 

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

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

 are, therefore, in a general sense Ethmophracti, but after a peculiar manner, entirely 

 different from that which characterises the older Spatangida3 comprised under this 

 appellation. They may be called Pei-issogonea "). There is not in any other genus of the 

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

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

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

 5, and the madreporite spreads its pores over the entire central area, without 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 Cassidulidaj *), in the Clypeastrida? '), of Tertiary origin. In these two 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 which they not 

 seldom are transferred. 



1) Etudes, p. 12, 50, PL XII, fiif. 103, 104; XXXII, Hir. 197—199. — A fossil species of this genus, 

 fi-om the Nuinmulitic strata of Westeru Sind, has been described as Hemiasier elonsatus Duncan and 

 Sladex, Mem. Gaol. Surv. India, Tert. etc. Ser. XIV; Vol. I, 3, p. 78, pi. XIX, iig. 7—15. Calcutta 1882. 



-) flsQiggSg, excessive, I'ov^, generative organ. 



3) Etudes, p. 81, pi. XV, fig. 133, 132. 



*) lb., fig. 130. 



5) lb., pi. XVI, fig. 135—139. 



