REVISION OF PALEOZOIC STELLEROIDEA. 119 



PROMOPAL^ASTER WYKOFFI (Miller and Gurley). 

 Plate 18, fig. 6; plate 19, fig. 2. 



Palxaster wykoffi MILLER and GURLEY, Bull. No. 12, Illinois State Mus. Nat. 

 Hist., 1897, p. 46, pi. 3, fig. 27. 



Original description. " Species medium size [R = 26 mm., r = 8 mm. 

 Width of ray at base 10 mm.] Rays pateliform; length about one 

 and a half tunes the diameter of the body; breadth of a ray at the 

 place of junction with the body, about two-thirds the diameter of 

 the body; obtusely pointed. 



"Marginal [inframarginal] plates wider than long and numbering 

 about 15 in the length of half an inch from the body [or about 28 

 in a complete column]. The [infra] marginal plates curve regularly 

 around the tips of the rays. The adambulacral range curves around 

 the ends of the rays within the marginal plates and consists of sub- 

 quadrate plates, wider than long. They are a little shorter than the 

 marginal plates, so that there are about 18 in the length of half an 

 inch [about 35 in a complete column]. There are 10 oral plates [oral 

 armature] at the junction of the adambulacral rows, which present 

 triangular extensions toward the center of the ventral cavity. A 

 single irregular axillary plate rests between the terminal marginal 

 plates and the angle formed at the junction of the adambulacral 

 plates [this description is inadequate, see beyond for the detail]. 

 The ambulacral plates have their greatest length across the rays, 

 thus providing a wide ambulacral furrow. Each plate is furnished 

 with a sharp ridge in the middle, extending from the middle furrow 

 to the adambulacral plates. 



"Our specimen exposes only the ventral side and no spines are 

 preserved. It is a well-marked and beautiful species." 



Formation and locality. Found in the Richmond formation "near 

 Madison, Indiana, by Charles W. Wykoff, in whose honor we have 

 proposed the specific name." The holotype is now in the Gurley 

 coUection of the University of Chicago (No. 6066). 



Remarks. Each interbrachial area begins orally with a pair of 

 elongate triangular basal adambulacrals (the oral armature pieces) 

 dis tally to which there are two pairs of adjoining adambulacrals 

 followed by a single axillary ossicle and usually one but also more 

 or less of a second pair of interbrachial marginal plates. P. wylcoffi 

 is therefore distinguished from P. exculptus in that it has orally to 

 the single interbrachial plate two pairs of proximal adambulacrals 

 instead of one as in the last-named species. These differences are 

 not decided, and it may eventually be seen when more well-preserved 

 material is at hand that they are either individual variations or due 

 to differences in age. 



