OLIGOPORUS. .V>7 



stage in Oligoporus is like the character of Lovenechinus at the mid-zone. Still higher up in 

 this specimen (Plate 50, fig. 9), isolated plates appear in the middle of the half-areas, fixing the 

 character of the type. It will be seen therefore that in Oligoporus danae the development of 

 the ambulacrum ventrally passes through a series of stages which are successively comparable 

 to the adult character of the three lower genera of the family as set forth diagrammatically in 

 text-figure 237, p. 231. 



The interambulacra are broad, gently curving in elevated ribs, not sharply inclined laterally 

 to the adradial sutures. There are nine columns of plates in each area at or above the mid- 

 zone in all specimens known. Ventrally, there are in the interambulacra (Plate 50, fig. 8) 

 two plates in the basicoronal row, three plates in the second, and four in the third row. In 

 area A the fifth column appears in the fifth row. In a fine specimen shown in Plate 49, figs. 

 4, 5, there are four interambulacra for study, which is more than is known in any other specimen. 

 In area A (Plate 50, fig. 1) there are nine columns at the mid-zone, and dorsally columns 1 and 2 

 drop out before reaching the apex, indicating senescence. In area C the ventral portion is 

 restored, as indicated by dotted lines. Assuming that this is correct, the fifth column origi- 

 nates in the sixth row, with a pentagonal plate and a heptagon on its right ventral border. 

 The sixth column originates to the right of the center in the ninth row, with a heptagon on its 

 left ventral border. The seventh column originates in the thirteenth row, with a heptagon on 

 its right ventral border. The eighth column originates in the sixteenth row, with a heptagon 

 on its right ventral border. The ninth column originates to the left of the center with a pen- 

 tagonal plate in the twenty-first row and with a heptagonal plate on its left ventral border. This 

 is a quite typical area, whereas the following present unusual features. In area E (Plate 50, 

 fig. 2) the ventral portion is also restored, and the adambulacral plates on either side show the 

 beveled sutural faces that extended under the ambulacra when in place. The fifth column 

 originates in the sixth row, with a pentagon on its right ventral border. The sixth column 

 originates in the ninth row, on the left of the center in a pentagonal plate, with a heptagon 

 on its left ventral border, and the seventh column originates in a pentagon in the center, in the 

 twelfth row. In this area two curious anomalies occur just above pentagon 6; in column 3 

 an aberrant pentagon, P, occurs, and, to compensate for its one side wanting, there is a heptagon 

 on its right side, immediately dorsal to pentagon 6. On the dorsal side of the aberrant pentagon 

 P, lies a heptagon, H. This plate compensates for the one side wanting in pentagon 7, but. in- 

 stead of lying next to it, is two plates removed. This is a very unusual and rare variant, but 

 emphasizes the mechanical requirements of the case, that, if a side is wanting in one plate, 

 there must be a side added to some other plate in order that all shall be either hexagonal or the 

 equivalent of the same. The eighth column in area E originates in the fifteenth row, on the 

 left of the center, with a pentagon, and a heptagon lies on its right ventral border. Column 

 nine originates in the nineteenth row, but the initial plate, instead of being pentagonal, is 



