PLATE 26. 

 Hyattechinus beecheri sp. nov. Page 297. 



''ig 1. Yale Mus. Coll., 323, holotype. Same specimen as photographs, Plate 24, figs. 5, 7, S, and Plate 25, fig. 5. X 1.1. 

 This unique specimen, though a sandstone mold of the interior, bears the impress of plates so that it can be drawn 

 spread out by the Loven method with very few gaps, part of which are filled in as plates with dotted lines in areas 

 3, 4, and 5. The gaps of the interambulacra laterally at the ambitus are due to the mechanical requirements in 

 making the drawing. This is the only regular sea-urchin known that is bilaterally symmetrical through an ambula- 

 crum and posterior interambulacrum, and the orientation is based on this bilateral symmetry- 

 Ambulacra broad, petaloid ventrally, narrow dorsally. Ambulacral plates ventrally near the median suture have 

 a series of pits caused by elevated spinose processes on the proximal face of the plates (Plate 24, fig. 6; compare Plate 

 3, figs. 12, 13; p. 61). Interambulacra have each 11 columns of plates. The primordial interambulacra] plates are 

 in the basicoronal row in each area; there are two plates in the second row, three in the third, and four in the fourth 

 row, each indicating the introduction of a newly added column. Column 5 originates in the fifth row in areas 1, 5, 

 and 4, but not until the sixth row in areas 2 and 3. The sixth column originates in the sixth row in areas 1, 5, and 4, 

 but not until the eighth row in areas 2 and 3. This seems to accord with the bilateral symmetry, the posterior 

 interambulacra! trivium developing more quickly than the anterior interambulacral bivium. After the sixth, the 

 succeeding columns come in with great rapidity, practically a new column being added in each row of each area until 

 the full number is attained at or near the ambitus. Dorsally, columns 1 and 2 and some additional columns in 

 each area drop out before reaching the apical disc. The peristome as far as indicated is covered with ambulacral 

 plates only, and the lantern is represented by casts of the alveolar cavities and inner faces of the teeth. 



