REPORT OF THE DIRECTOR I915 I35 



Plates I and 6 offer proof positive that in U . medusa there 

 were no spines on the outer faces of the cover plates. These are 

 the faces Schuchert evidently designates as abactinal. A portion of 

 these faces are covered by the marginals and the uncovered por- 

 tions are " abactinal " only when the arm is open. The true actinal 

 face is here called the apical and this, as we have seen, is articulated 

 with the floor plates. The spines which Schuchert credits to the 

 "abactinal" side of the cover plates in U. grandis (1915, 

 page 181) and illustrated in his plate 30, figures i and 2, would 

 seem to belong to the marginals. Figure i of this plate is veiy sug- 

 gestive of such an origin. The longitudinal grooving of these spines 

 here shown is doubtless a line of contact of two spinelets. These 

 spinelets (as measured from the figures) were nearly .8 mm long 

 and the pedicels about .13 mm in diameter. This diameter is but 

 slightly in excess of the .12 mm which we gave for the pedicels on 

 the marginals of U. medusa. An examination under gum 

 mounting would probably more clearly reveal the character of the 

 articulation of the spinelets with their pedicels. 



Figure 7 of Schuchert's plate 30 shows three of his " five very 

 stout, short pointed spines (tori) " which " are inserted inside of 

 the oral armature." If these are spines, as represented, this speci- 

 men can not be congeneric with the plesiotype of U . p u 1 c h e 1 1 a . 

 I should interpret these as simply the paired first cover plates as 

 seen In our plate 10. 



Schuchert's figure seems to show a detached pedicel whose origin 

 was from the axillary inframarginal in upper right interradius. 

 We also saw evidence for such a piece in our plate 10. Sihiilai* 

 spinous processes are shown in the interradii of Schuchert's figure 

 of U. girvanensis. Here, however, their relation to the 

 first paired cover pieces is more clearly shown. 

 ■ Beyond noting the loosely imbricated arrangement of the 

 ambitals, an adaptation allowing of the opening and closing of the 

 cover plate columns, we shall in this paper have nothing more to 

 say concerning the apical skeleton of Urasterella. It may be of 

 interest, however, to compare briefly our conception of this genus 

 with Protopalaeaster as known to us through P. narrawayi. 

 Schuchert includes Protopalaeaster under Hudsonaster and says of 

 the genus (1915, page 40) "In Hudsonaster we have the most 

 primitive known starfish." Spencer, on the other hand (1914), 

 separates a new genus " Eoactis " from the Urasterella group and 

 believes that it shows one of the simplest of mouth frames. 'We 

 shall here give only some of the more interesting differences between 

 these genera. For this purpose we shall accept the interpretation of 



