96 University of Texas Bulletin 



DESCRIPTION: 



SUPERO-MARGINAL PLATES: On each aboral interarc there are 

 fourteen supero-marginal plates excluding the terminal plate, which is 

 common to each two adjacent arcs. These plates, though separated, seem 

 well preserved, and in this individual as in the type, lack ornamentation. 

 It is entirely possible that better material will show superficial structure 

 on the plates since most related species show it. In shape the plates are 

 trapezoidal or cuneiform, as described for the type. They are tallest 

 and widest at the center of the arc and decrease in size to the tips of the 

 rays. The marginal plates of this species are relatively large and are 

 taller than broad ; in Pentagonaster browni Weller, they are broader than 

 tall, and are relatively smaller. 



PARAMARGINAL PLATES : These are a row of minute, nearly equal 

 and similar subquadrate plates lying apposed to the inner edge of the 

 marginal plates. Only a few, near the tips of the rays, are preserved in 

 this individual; they apparently belong to the aboral surface. 



ADAMBULACRAL PLATES : These plates, as described in the type, 

 are mostly quadrate and are of about the same size as the other interior 

 oral and aboral plates (about .5 mm. diameter). This individual has 

 along one radius two nearly apposed rows of plates that belong to this 

 class; they diminish in size somewhat toward the tip of the ray. 



CIRCUMORAL PLATES: There is a group of plates massed around 

 the mouth region, which are in part oral plates and in part dislocated 

 central aboral plates. They are rounded and larger than the other interior 

 plates, and a few are elongate. In the absence of better material, they can 

 not be described more accurately. 



INTERMEDIATE PLATES : There are a few continuous radial rows 

 of plates lying on the radii and extending from the center of the disk to 

 the tips of the rays. These are apparently aboral plates. They have a 

 rounded, roughly quadrate to hexagonal shape, and are small, equal and 

 similar. The madreporite and ambulacrai details are not visible. 



RELATED SPECIES : Pentagonaster browni Weller (Fox Hills form- 

 ation, Lander, Wyoming) is described as having 16 supero-marginal plates 

 which are relatively smaller and less tall than in this species. The inter- 

 radial arcs of the Wyoming species are also deeper than in our material. 

 P. lunulatus Woodward and P. megaloplax Sladen (Upper Chalk, Senonian) 

 differ from the Texas species in having punctate or papillose marginal 

 plates, in the concavity of the interarcs, and in the number of marginal 

 plates. P. robustus Spencer (Upper Chalk) differs in the same charac- 

 ters and in the meeting of the supero-marginal plates along the midline 



