350 ROBERT TRACY JACKSON ON ECHINI. 



seventh column originates in the sixteenth to the nineteenth row in the several areas. In area 

 A it appears in the eighteenth row; in C in the nineteenth row; in G in the seventeenth row ; 

 and in areas E and I in the sixteenth row. It is worth noting that in areas E and I, in which 

 the seventh column originates earlier than in the others, column 6 also originates earlier, showing 

 an acceleration in both features. Also, in area C columns 4, 5, and 7 all originate later 

 than in the other areas, showing a corresponding retardation of development in those several 

 features (p. 50). Dorsally in part, as shown in area C, the adambulacral columns 1 and 2 

 drop out before reaching the apical disc, indicating senescence. 



OLIGOPORUS Meek and Worthen. 



Oligoporus Meek and Worthen, 1860a, p. 472; 1866, p. 247; (pars) Loven, 1874, p. 42; Duncan, 1889a, 



p. 16; (pars) Jackson, 1896, p. 239; (pars) Lambert and Thiery, 1910, p. 121. 

 Melonopsis Meek and Worthen, 1866, p. 249. 



The test is spheroidal, with more or less strongly developed melon-like ribs. Ambulacra 

 at the mid-zone with four columns of plates which consist of two columns of wide occluded and 

 two columns of narrower demi-plates, with, in addition, scattered isolated plates in the middle 

 line of each half-area, but not of sufficient frequency to constitute an additional column. The 

 wide occluded plates are roundly arched upward in the center. The pore-pairs are multiserial, 

 one series in each column of demi- and occluded plates, and, in addition, a broken series in the 

 scattered isolated plates. 



The interambulacra are wide, with from four to nine columns of plates in an area in the 

 known species. The columns are composed of pentagonal adradial and hexagonal median 

 plates. There are ventrally two plates in the basicoronal row, above which additional columns 

 come in as usual in the family. There is no imbrication in ambulacral or interambulacral 

 plates, but the ambulacrals bevel over the adradials laterally. Ambulacral and interambula- 

 cral plates bear small secondary tubercles with small tapering spines. Peristome unknown. 

 Apical disc imperfectly known, but doubtless similar to that in Melonechinus with insert, 

 imperforate oculars and high, wide genitals bearing several genital pores each. Lantern known 

 partially in 0. danae, from which it is seen that the lantern was inclined, pyramids wide-angled 

 with moderately deep foramen magnum as usual in the Perischoechinoida. 



The type species of the genus is Oligoporus danae (Plate 49, figs. 4, 5; Plate 47, fig. 13; 

 Plate 50, figs. 1-12). 



Oligoporus differs from Lovenechinus in that it has scattered isolated plates in the ambulacra 

 in addition to the four columns of demi- and occluded plates. Thus it is a step in advance in 

 the evolution of this important structure. In the interambulacra it is also more advanced in 

 that the highest species (danae) has nine columns of interambulacral plates in an area, whereas 



