190 R. T. JACKSON — STUDIES OP PAL.EECHIN0IDP:A. 



Rhoechimis, (PaLxchinus^) and Melonites, having twice the number of am- 

 bulacral plates of the former and less than the latter. His view is fully 

 substantiated by the development. 



In Oligoporus the development of the two columns of plates in each 

 half ambulacrum from the very early condition of one column of plates^ 

 as seen ventrally, ma}^ be compared to interlocking one's fingers and then 

 gradually pulling them apart. It is seen from studying the figures that 

 the two columns of plates, a, 6, at the ventral border of Oligoporus (plate- 

 6, figure 25) together correspond to the 4 columns of plates, a, a', b\ b, in 

 adult Oligoporus ; also the}^ are the homologue of the 4 columns, «, a\ h' b, 

 at the ventral border of Melonites (plate 2, figure 4) and in adult Melonites 

 (plate 2, figures 2 and 4, and plate 4, figure 19) they correspond to the two 

 large median columns of ambulacral plates, a\ I/, together with the twa 

 small lateral columns, a, b. We have then the interesting morphological 

 deduction, figure 1, page 191, that the 4 columns of Oligoporus are the- 

 equivalent of the two median and two outer columns of ambulacrals in 

 Melonites^ and that these 4 columns of each genus are traceable to and 

 directly derivable from the two columns of ambulacrals in types like 

 RJioechinus (plate 7, figure 37) and the two plus columns in PaLneechinvsf 

 (plate 7, figure 39), or earlier still to the two columns existent in the 

 ancient i:)rimitive tj^pe Bothriocidaris, figure 4, page 234. These 4 columns 

 of Oligoporus and Melonites also appear to be the morphological equiva- 

 lent of the two columns of ambulacrals seen in Archxocidaris and allies; 

 also modern Cidaris and related forms. 



Turning to the consideration of the ambulacrum of adult Oligoporus 

 danx (plate 6, figure 30), it is seen that there are 4 columns, a, a\ b\ b, of 

 well developed, clearly defined plates. It is also seen at certain points 

 that accessory plates exist between the primary ambulacral i)lates and 

 close to the middle of each half area. These accessory plates are not 

 regularly distributed excepting in so far as occupying a median position. 

 It seems that in these occasional plates we can see the foreshadowings of 

 additional columns of plates, which, as shown in Melonites (plate 2, figures 

 4 and 2), originate in just this same median position between the outer 

 and median column of each half ambulacral area. 



The morphological and genetic relations of the ambulacral plates in 

 these genera are expressed in the accompanying diagram, figure 1, which^ 

 with the foregoing description, fully illustrates itself. It is believed that 

 this figure expresses the correct ph3dogenetic relations of the included 



* The species of Palccechinus have been divided between the sjenera Rhoechinus and Palceechinus 

 by Duncan (8) since Etherid^e's paper was puljlished. 



t In Palceechinus there are not 4 fully developed columns, as in Oligoporus. It is really a transi- 

 tion condition, and for convenience in this paper the number of ambulacral columns are designated 

 as two plus. See, also, description of this genus, page 205, and the systematic table facing page 242- 



