146 BULLETIN 100, UNITED STATES NATIONAL MUSEUM 



STICTOCYCLUS, new genus 



Valve circular, almost flat until, at the apparent rim, it bends ver- 

 tically downward at right angle to the surface of the disk to form a 

 deep flange about one-third the disk radius in width, thus making 

 each valve a shallow circular half box, the two valves joined by the 

 girdle thus forming a thick frustule ; the markings of the flange are 

 the same as and continuous with those of the disk; in the middle of 

 the valve is a small irregular central area, bearing a few minute 

 scattered beads ; from the center proceed 50 to 80 sharp but narrow 

 radii extending to the apparent rim and over it to the lower edge of 

 the flange; the spaces between the radii are ornamented with small, 

 densely set beads only obscurely arranged in rows ; a comparatively 

 large but not typical pseudonodule is placed in from the apparent rim 

 about one-fourth to one-eighth the radial distance ; this is not a glisten- 

 ing body like the true pseudonodule of Actinocyclus, because it is not 

 due to a lenticular thickening of the silica, but is like the similar spot 

 in Roperia or the ocellus of Pseudo-Auliscus; the side (or girdle) view 

 shows the continuity of the vertical flange with the rest of the valve 

 and the flatness of the surface of the disk portion. Markings of the 

 girdle unknown. Only the one species following is so far known. 



This exquisitely ornate and delicate diatom, figured and described 

 under the name u Actinocyclus stictodiscus, ,, strangely resembles both 

 these dissimilar genera, a fact indicated by the new name here pro- 

 posed; but it can not be assigned to either one without introducing 

 objectionable modifications. The vertical flange continuous with the 

 disk, and its pseudonodule exclude it from Stictodiscus; the dissimi- 

 larity of this pseudonodule from that of Actinocyclus, the verticle 

 flange extension of the disk and the radial costae exclude it from 

 Actinocyclus. It certainly is more unlike either of these than Roperia 

 is unlike Actinocyclus or Araclinoidiscus unlike Stictodiscus. Its side 

 view, here illustrated for the first time, will make this clear. The 

 suggestion of Van Heurck, that it represents a separate genus, is 

 therefore here adopted. 



STICTOCYCLUS VARICUS, new name 



Plate 32, figs. 1-2 

 (Van Heurck, Synopsis, pi. 118, fig. 4, misnamed.) 



General characters as in the genus. The valves are delicately thin 

 and elastic. The radii adjacent to the pseudonodule on either side 

 are slightly bowed around it. 



Diameter to apparent rim, 0.130 to 0.199; width of flange, 0.035 to 

 0.053; width of sectors at apparent rim, 0.007 to 0.008; diameter of 

 pseudonodule, 0.008 mm. 



Type.— Cat. No. 43G80, U.S.N.M. 



