336 ROBERT TRACY JACKSON OX ECHINI. 



(1873) are: height and breadth about 3.73 inches each, width of the ambulacra about 0.6 of 

 an inch, width of the interambulacra about 1.06 inches. The height bf the largest interambul- 

 acral plates is 0.26 of an inch, width 0.40 of an inch, and thickness 0.25 of an inch. In a 

 splendid specimen in the U. S. National Museum (Plate 37, fig. 3), the ambulacra at the mid- 

 zone measure 17 mm. in width, interambulacra 40 mm. The height as estimated is something 

 less than the width. Two specimens in the Museum of Comparative Zoology (Plate 37, figs. 

 1, 2) have about the same proportions, but one of these (fig. 2) is more massive, the inter- 

 ambulacrum measuring 50 mm. in width. The very thick plates are seen well in these two 

 specimens. The plates in the middle of the ambulacrum and the interambulacrum (Plate 

 37, fig. 1) measure 7 mm. in thickness, but at the adradial suture they are only 3 mm. thick. 

 The type according to Meek and Worthen is an internal siliceous mold, so that as this was a 

 unique specimen the thickness of the plates was not shown. 



The ambulacra are relatively broad, with four columns of plates at the mid-zone, two of 

 narrow demi- and two of wide occluded (Plate 38, figs. 5, 6). The demi-plates bevel strongly 

 over the adradials, the occluded medially rise into a high rounded ridge, in the center of which 

 the plates attain a thickness of some 7 mm. The inner face of the test (Plate 38, fig. 9) presents 

 no corresponding elevation, but rather is on one continuous curve of a practically uniform arc 

 throughout. The occluded plates laterally intercalate in a tongue-like end between the elon- 

 gated and narrowed corresponding tongues on the inner ends of the demi-plates. The inner 

 end of the tongue of the demi-plates in some cases is superficially almost or entirely separated 

 from the bodj^ of its plate by the expanded tongues of the occluded plates (Plate 38, fig. 6). 

 Pore-pairs are situated in the outer tongues of the occluded plates and close to the marginal 

 suture in the demi-plates. Occluded plates bear double alternating rows of small secondary 

 tubercles, the outlines of the plates waving in correspondence with their position. The demi- 

 plates bear usually only a single tubercle on the rounded inner end of the tongue. About five 

 or five and a half demi-plates equal the height of an adradial. 



The interambulacra are roundly curved and laterally dip down to the adradial sutures 

 where they bevel under the ambulacrals (Plate 38, fig. 9). There are five columns of plates 

 at the mid-zone in all areas of the specimens seen. The thickest plates are in the center of the 

 area, and the thinnest are the pentagonal adradials. All five columns extend throughout the 

 areas as far as observed; but the ventral developmental stages are not preserved in known 

 specimens. The surface of the plates is covered with numerous small secondary tubercles, 

 similar "to those of the ambulacral areas. 



According to Meek and Worthen, the apical disc in the type specimen is similar to that of 

 Melonechinus muUiporus, the ocular and genital plates being much the same. No pores were 

 seen in the oculars, and the genitals, as far as ascertained, had four or five pores each. Two 

 of the genitals have five pores each, one apparently five, and two apparently four pores each. 

 The apical disc is not preserved in specimens I have seen. 



