342 ROBERT TRACY JACKSON ON ECHINI. 



are broadly insert, but oculars D and F are exsert. This is the only case of exsert oculars seen 

 in the species. Ventrally, the oculars bear each a small siliceous plug, representing a cast of 

 the ocular pore, which probably did not reach the surface, as such have not been seen on the 

 surface in any species of this family. The oculars ventrally cover the ambulacra and laterally 

 the interambulacra in part on either side. The apical disc measures 7 mm. in diameter and is 

 proportionately about 21 % of the diameter of the test. This is proportionately large for the 

 species and family, but evidently, as in Recent Echini, the apical disc is proportionately larger 

 in young individuals (p. 87). In this dorsal view the single plate representing the sixth column 

 is clearly seen in area I. 



In Mr. Braun's specimen (Plate 39, fig. 3; Plate 41, fig. 3), from Joplin, Missouri, we 

 have an internal siliceous mold about 58 mm. in diameter and nearly spherical. It has five 

 columns of plates in each interambulacral area, but dorsally the fifth column is strung out, 

 and in areas A, C, and E fails to reach the apical disc (Plate 41, fig. 3). Oculars are all insert, 

 and ventrally cover the ambulacra and laterally the interambulacra in part on either side. The 

 genitals are wide, high, and have four or five genital pores each. Dorsally, in this internal 

 mold, ambulacral plates are all primaries, with pore-pairs uniserial; farther ventrally they are 

 primaries and occluded, with pore-pairs biserial (compare Plate 42, fig. 3). 



A magnificent specimen in the Museum of Comparative Zoology Collection 3,128, from Webb 

 City, Missouri, is one of the finest Palaeozoic Echini seen. It is an internal siliceous mold, 

 and is represented in ventral and dorsal views in Plate 39, figs. 4, 5. The boundary of each 

 plate is represented by a siliceous ridge, so that, if plates have been separated, there is an inde- 

 pendent bounding ridge for each plate, as seen well in area E (Plate 39, fig. 5). Siliceous plugs 

 represent the ambulacral and ocular pores and delicate siliceous tubes the casts of the larger 

 genital pores. The specimen is somewhat compressed, but averages about 95 mm. in diameter. 

 It is not only full grown, but is an old-age individual, as indicated by the dropping out of columns 

 dorsally, as described below. 



So nearly .complete is this specimen that I am able to represent it spread out by the Loven 

 method (Plate 41, fig. 1), with the restoration of only a few plates ventrally, as indicated by 

 dotted lines. The ambulacral plates at the mid-zone are demi- and occluded, with pore-pairs 

 biserial, but situated on either side in the middle line of each half-area (compare Plate 42, figs. 

 2, 3; Plate 43, figs. 4, 5). The ambulacral plates near the peristome, which were built in 

 youth, and the young plates dorsally, near the ocular, as a localized stage, are primaries, 

 crossing the half-areas, and with pore-pairs uniserial (compare Plate 42, figs. 1, 3). 



In the interambulacra (Plate 41, fig. 1) there are two plates in the basicoronal row, in part 

 restored, three plates in the second row, also partially restored, four plates in the third row, 

 restored in two areas. The fourth column originates on the left of the center hi areas A and 

 C, but apparently (in part restored) to the right of the center in E, G, and I. There are a good 



