REPORT OF THE DIRECTOR I915 I23 



which Schuchert gives for the madreporite of Pet raster 

 speciosus (Miller and Dyer) in 1915, plate 27, figure 3, only 

 it is much smaller. 



Schuchert^ (1915) mentions no heavy first floor plates for any 

 species of Urasterella. In his plate 2,"], figure 7, he presents 

 a camera lucida drawing of part of the proximal, oral area of a 

 ray of U. grandis (Meek) of which he says (page 298) " The 

 complete oral armature is preserved." The first floor plates, as here 

 represented, are no heavier than those which follow them and 

 instead of together forming a medial toothlike projection, they 

 present a very marked concavity next the oral aperture. Schu- 

 chert's plate 28, figure 5, presents the first floor plates of U . 

 girvanensis Schuchert, as no larger than others in the same 

 ray and (in the long arm) recessed next the oral cavity. Spencer 

 (1914, plate I, figure 4) gives essentially the same form for these 

 plates. These facts should be sufficient excuse for the details here 

 given. The markedly different character of the peristomial floor 

 plates, shown in our figures, and the essentially different form 

 they give to the border of the oral cavity may be generic characters. 



Arm Floor Plates 

 The arm floor plates are placed with their inner ends thrust 

 toward the mouth and they are also imbricated. Near the per- 

 istomial ring the proximal face of each plate, when viewed orally, 

 slightly overlaps the distal face of its older neighbor. This con- 

 dition is well shown in plate i, figure i, arm A. In plate 2, figure 



1, the apical aspect of the second floor plates, or those next the 

 peristomial ring, would indicate that the overlap was confined 

 largely to the inner ends of the plates. The majority of the floor 

 plates, however, are tipped the other way and slightly overlap their 

 younger neighbors when viewed orally. See plate i, figure 2, and 

 plate 6, figure i. 



In U. pulchella (plate 10, ray e) the seven and one-half 

 floor plates, following the first, form a line 3 mn7 long and the long 

 axes of these plates measure 1.3 mm. In U. medusa (plate 



2, figure i) there are but six floor plates in a similarly placed line. 



1 As Schuchert's important recent work (1915) must be accepted as a 

 standard, we shall make 'frequent reference to it. Our dissection, gum 

 mounting, photomicrographs and stereograms have revealed mucli that is 

 new concerning the genus Urasterella. In some instances it will make 

 necessary a modification of Schuchert's description of this genus ; in others 

 it will corroborate certain very important deductions of his. 



