358 ROBERT TRACY JACKSON ON ECHINI. 



tetragonal, and to compensate for the two sides wanting, there are two heptagons on its right 

 and left ventral borders respectively. Area I (Plate 50, fig. 3 ; compare Plate 49, fig. 5) is viewed 

 from the interior of the test. The ventral portion is restored. Assuming that this is correct, 

 the fifth column originates in a pentagon in the sixth row, with a heptagon on its left (right as 

 seen from exterior) ventral border. The sixth column originates with a pentagon in the ninth 

 row with a heptagon on its right (left as seen from the exterior) ventral border. Column 7 

 originates with a pentagon in the twelfth row, and a heptagon is on its right (left as seen from 

 the exterior) ventral border. The eighth column originates with a pentagon in the fifteenth 

 row, with a heptagon on its left (right as seen from the exterior) ventral border. Finally, the 

 ninth column originates in the twenty-third row, and, as in the same column in area E, the 

 initial plate of this column is tetragonal, and to compensate for the two sides wanting, has two 

 heptagonal plates on its right and left ventral borders (pp. 369, 391). 



The detailed structure of the interambulacra in the type specimen, and also in a specimen 

 in the Museum of Comparative Zoology Collection, 2,997, is figured and discussed in my 

 earlier paper (1896, pp. 193, 197, Plate 6, figs. 31, 34). In both there are nine columns of 

 interambulacral plates, the ninth originating late, considerably above the mid-zone. In this 

 species both ambulacral and interanibulacral plates bear numerous small secondary tubercles, 

 with small tapering spines about 5 mm. long. A specimen from Edwardsville, Indiana, in the 

 Museum of Comparative Zoology, no. 3,187, shows clearly the ventral developing stages of 

 ambulacral and interambulacral areas, also spines. 



A genital plate is high and wide with three genital pores (Plate 50, fig. 6). The outline 

 of this plate presents straight lateral sides for sutural contact with the oculars, so that it may be 

 inferred, although they are not preserved, that the oculars separated the genitals and were insert, 

 as indicated by dotted lines in the sketch. The lantern as shown by two half-pyramids (Plate 

 50, figs. 11, 12) is inclined, pyramids wide-angled, with moderately deep foramen magnum. 

 The pyramidal suture extends to the base of the foramen as usual, and the dental slides, as seen 

 in internal view, extend to about the level of the foramen. The lateral wing of the pyramids 

 is wide (Plate 50, fig. 11) and has the remains of plicate ridges for the attachment of inter- 

 pyramidal muscles. 



Oligoporus danae is the type species of the genus. The original locality given by Meek 

 and Worthen (1866, p. 251) is Keokuk Group, Lower Carboniferous, Jersey County, Illinois; 

 also Warsaw, Illinois. The type is said to be in the Worthen Collection, now in the University 

 of Illinois at Urbana, Illinois. 



Keokuk Group, Lower Carboniferous, Adams County, Illinois, Museum of Comparative 

 Zoology Collection 3,142; Warsaw, Illinois, Museum of Comparative Zoology Collection 2,997; 

 Keokuk, Iowa, Museum of Comparative Zoology Collection 2,998, figured by R. T. Jackson 

 1896, Plate 6, fig. 30; Salem, Indiana, Museum of Comparative Zoology Collection 3,143 



