RHYNOBRISSUS PLACOPETALUS. 215 



rostrate and one tridentate, no globiferous or triphyllous. The rostrate had 

 valves about .30 mm. long, with the base .08 mm. and the blade .03-.04 mm. 

 wide. The tridentate had moderately wide, leaf-shaped valves about .18 mm. 

 long. In the holotype, I could unfortunately find no ophicephalous pedicel- 

 lariae, nor were there any globiferous. Rostrate and tridentate are abundant, 

 chiefly near subanal fasciole and in ventral ambulacra posteriorly. Neither 

 is at all distinctive. The rostrate resemble those in the small specimen but 

 the valves are .25-80 mm. long. The tridentate have straight rather narrow 

 valves, .25-50 mm. long; the small ones have the blade widest at middle but 

 in the large ones it is widest near tip. A few triph}dlous pedicellariae were 

 found; these have trowel-shaped valves about .10 mm. long. 



Rhynobrissus placopetalus. 



Rhinobrissus placopetalus A. Agassiz and Clark, 1907. Bull. M. C. Z., 60, p. 256. 



Plate 152, figs. 1-4. 



Length 14 mm.; width 12 mm. at mouth, where test is widest; height at 

 peristome, 8 mm.; at posterior end, 10 mm. The general form of the test is 

 well seen in the figures. The apical system is subcentral in position and shows 

 four genital pores but the right anterior is very small. The petals are subequal, 

 a little sunken, 4 mm. long with 13 or 14 pairs of pores on each side. Ambula- 

 crum III is not petaloid, sunken or in any way recognizable. The peripetalous 

 fasciole though slender is well marked. The periproct is much higher than 

 wide and is situated near the top of the truncate posterior end of the test. The 

 subanal fasciole is very distinct and the anal branches, one on each side of the 

 periproct, are nearly as well marked. The subanal plastron is almost vertical 

 in position and is distinctly convex, but it can hardly be said to be protruding. 

 It is notable that only two ambulacral plates on each side enter into the compo- 

 sition of the plastron (PI. 152, fig. 4) and there are no large tube-feet there. 

 The sternum is narrow, about two and a half times as long as wide, and strongly 

 carinate (PI. 152, figs. 2, 3). The ambulacral plates of the oral surface are 

 not smooth but seem to be slightly sculptured in rather strong contrast to the 

 adjoining interambulacral plates; this may be, however, an indication of im- 

 maturity and not a specific character. The most striking features of the test 

 are the shutting out of interambulacra 1 and 4 from the peristome, the small 

 size of the primordial interambulacral plate in each of these areas, and the dis- 



