Classificatory Position of Hemiaster elongatns. 233 



perforated generative plates. But there is a little more poro- 

 sity of the space between the perforated plates than is drawn 

 by Loven. 



II. 



In considering the distinctions between Palceostoma and 

 Palceotrojnis it is necessary to quote from Prof. Lovcu's work 

 on Pourtahsia^ p. 79. He writes : " Palceostoma mirahilej 

 Gray, deviates in a strange manner from nearly all the rest of 

 the Spatangidaj 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, b 2, 

 and b 3 ] hy the very irregular interradium 5, and by the 

 pentagonal peristome with its five valves, and from Palceo- 

 troj}ics in particular, by its distinct petals and by the absence 

 of a subanal, the presence of a peripetalous fasciola. But 

 with all this, Palceostoma offers a calycinal system evidently 

 constructed upon the same plan as in that genus," i. e. Palceo- 

 tropus &c. Then follows a summary of the calycinal 

 features, which have been given already. But that part of 

 the description which relates to the raadreporite is given so 

 definitely by Loven that it may well be given again, espe- 

 cially as the statement affects this reply in a very decided 

 manner. Loven notices that the two mammiform and pro- 

 minent sexual outlets are placed transversely against the 

 interradials 1 and 4, so as to prevent the retrograde passage 

 of the madreporite. 



The point in tlie argument that we wish to make after this 

 comparison of the two genera is exceedingly simple, and it is 

 tliat Prof. Loven admits certain structural dilierences in Palceo- 

 stoma to be of sufficient importance to override the similarity 

 of the calycinal structures which the genera have in common. 



It is clear that, like any other naturalist, Loven admits 

 that although two forms possessing some similar and important 

 structures are closely allied, yet if there are some otlier and 

 very decided structural differences between the forms they 

 cannot belong to the same genus. Palceostoma is not the same 

 genus as PalceotrojyiiSj because, although there are some points 

 in common about the apical disk, there are others, elsewhere, 

 which are not so. The same elementary reasoning will apply 

 to any other genera which may have some, but which may 

 not have all, of the principal characters of Palceostoma. In 

 concluding tliis part of our reply, we can state that Hemiaster 

 elongatus, nobis, having some structures in common with 

 Palceostoma^ possesses many more which do not characterize 

 the genus in which Prof. Lovdn would place it. The 



