24] 
rounded, partitions sunken, flexuous, with a subtuber- 
eular pellucid elevation in the centre. Minute. Diffused 
but not common. 
P. CrassutA.—Navtiwus. C. Mont. Sup. p. 79. t. 
18. f. 2.— Walk. t. 4. f. 70.—F._ p. 229.—Turt. D. 
p. 119.—D. p. 343.—Inn. T. 8. p. 117.—Depressed, 
umbilicated, opaque brown, with numerous  close-set 
joints, shewing part of the interior volution. with nu- 
merous close-set elevated joints ; sides similar ; mouth 
a little oblique, scarcely clasping the body and furnished 
with a syphon. Hnglish, very rare. 
P. GuitieLMiIne. Macg. Aberd. p. 315. Orbi- 
cular, disciform, greyish white, considerably compressed, 
the sides equally convex, bevelled to a thin subcarinate 
margin ; the cells of the last whorl about fifteen, elevated, 
narrow, convex, with the concave interstices transversely 
sulcated; the centres elevated; the end moderately 
raised, semilunar, convex, with its sides embracing the 
previous turn, 0,02. Aberdeen. 
P. Crenuata. Macg. Aberd. p. 316. Roundish, 
discoid, pale bluish grey, the sides somewhat convex, 
the margin rounded, the centres slightly depressed ; the 
rays formed by the cells about twenty, curved, convex, 
K 
