228 



ROBERT TRACY JACKSON ON ECHINI. 



such as are present in Oligoporus (Plate 50, fig. 7), this being the distinction between the two 

 genera. In the interambulacra of Lovenechinus in the known species there are from four 

 to seven columns of plates in an area. Of the several species the most completely known 

 are Lovenechinus missouriensis and L. septies (Plates 39-45 and 46, fig. 4), (p. 324). 



The genus Oligoporus Meek and Worthen is characterized by the fact that the ambulacrum 

 at the mid-zone has four columns of plates and, in addition, in the middle line of each half-area 

 isolated plates at intervals, but not in a continuous series so as to make an additional column 

 (Plate 50, fig. 7). This structure is distinctly intermediate between that of Lovenechinus and 

 the simplest species of Melonechinus. The youthful Oligoporus (Plate 50, fig. 8) has exactly 

 the structure of adult Lovenechinus, and the youthful Melonechinus as a second stage in devel- 

 opment (Plate 56, fig. 3) has exactly the structure of adult Oligoporus. In the interambula- 

 crum of Oligoporus there are from four to nine columns of plates in an area according to the 

 species. All known species are American, from the Lower Carboniferous, and the best known 

 is Oligoporus danae (text-fig. 237; Plate 47, fig. 13; Plate 49, figs. 4, 5; Plate 50), (p. 350). 



The genus Melonechinus Meek and Worthen has many species with a very wide range of 

 characters. The ambulacrum at the mid-zone has from six to twelve somewhat irregular col- 

 umns of plates. The two median columns of occluded plates that extend from the middle 

 of the ambulacrum outward are wider than other plates of the area, and are principally involved 

 in the more or less high ridge that usually marks the central line of the ambulacrum. The 

 demi-plates which extend from the interambulacrum inward do not differ externally from 

 others of the area except the occluded plates, with the exception of the fact that they bevel 

 over the interambulacra. On the outer face of the plates the pore-pairs are surrounded by 

 a peripodium which occupies that part of the plate nearest the next adjacent interambula- 

 crum. On the other hand, on the internal face of the plates the pore-pairs are very near the 

 center of the isolated plates but in demi- and occluded plates, the pore-pairs lie nearer to the 

 center of the half-area than to the middle of the plates (Plate 56, figs. 4, 5) . The ambulacrum 

 of Melonechinus on the ventral border has four columns of plates like the typical character at 

 the mid-zone of Lovenechinus. Passing from this area dorsally, scattered isolated plates 

 appear, the generic character of Oligoporus; further dorsally the isolated plates are more fre- 

 quent and fall into columns which are more or less in number according to the species in hand. 

 This is shown in text-fig. 245, p. 382; Plate 56, fig. 3; Plate 57, fig. 1 ; and others. In one area of 

 a specimen of M . multiporus, as a regressive variant there are simple primary plates ventrally 

 (Plate 57, fig. 3). This shows that while the genus typically has four columns ventrally, it 

 may revert to two columns as a variation, repeating the character seen at this area in the lower 

 genera of the family, and likewise the character seen at the mid-zone in Palaeechinus, the 

 lowest genus of the family. In the dorsal region of Melonechinus near the apical disc, in the 

 region of young last added plates, the ambulacrum is also simpler. As few as two plates may 



