PLANAMASTRA. 129 



Genus PLANAMASTRA nov. 



Amastrinae of discoidal shape, witli flattened embryonic 

 whorls, like those of Pterodiscus, the umbilicus very broadly 

 open. Peristome unexpanded, the columellar margin dilated, 

 without a colmnellar lamella at any stage of growth. Vivi- 

 parous. 



Type : P. digonophora. Distribution : Oahu. 



The jaw of P. digonophora is very thin, deeply arcuate, 

 smooth except for a few wide plaits or flat ribs in the median 

 part. 



Teeth of P. digonophora. 



The radula of P. digonophora has 11, 7, 1, 7, 11 teeth. 

 Those of the median field stand in nearly straight transverse 

 rows, but at the sides the rows of marginal teeth bend rapidly 

 forward. The centrals are narrow, less than half the width 

 of the adjacent lateral teeth, and bear a single small cusp. 

 The lateral teeth are square, bicuspid with a large mesocone 

 and small ectocone as usual. The marginal teeth .are short 

 and broad, with the mesocone oblique, and the ectocone split 

 into two, three or four minute, acute denticles. 



The embryonic shell of P. digonophora (pi. 25, figs. 4, 5, 6) 

 is much smaller than in Pterodiscus, having a diameter of 

 1.8 mm. with 1% whorls. The top is like that of Pterodiscus, 

 the first whorl being smooth except for some very faint radial 

 ripples and a microscopic crinkling of the cuticle. On the 

 second whorl the wrinkle-striation becomes coarser and re- 

 tractive, and there are a few very faint spirals. The first 

 whorl is slightly convex, the second flatter. 



Planamastra and Pterodiscus have in common a flat-topped 

 embryonic shell. They doubtless diverged from a common 



