124 CATALOGUE OF PHANEROPNEUMONA. 
XIX. LITHIDION. 
Operculum subcircular, shelly ; whorls rather rapidly enlarged, 
with a strong, convex, subcentral keel and simple edges. Shell 
depressed, broadly umbilicated, open. Aperture rather circular. 
Peristome simple, often thickened. 
Lithidion, Gray, Cat. Cycl. p. 35; Pfr. in Z. f. Mf. 1851, p. 156; 
Consp. p. 28; Pneum. Mon. p. 176. 
Cyclostoma sp., Sow. Petit, de. 
1. LivHii0on SULCATUM. B. M. 
Shell convex-depressed, solid, girdled with rather sharp raised spi- 
ral ridges, slightly shining, blueish-white ; spire somewhat raised, 
with the apex blueish, mucronulate ; whorls 5, moderately con- 
vex, last rounded, deflected anteriorly ; umbilicus wide, smooth 
internally ; aperture very oblique, rather circular, brown inside; 
peristome rather thickened, slightly expanded, with the margins 
approximate, right margin sinuate. Height 5, greatest breadth 
10, least br. 9 mill. 
Cyclostoma lithidion, Sow. Thes. n. 61, p. 111, t. 31, f. 262; Pfr. Lc. 
n. 160, p. 150, t. 20, f. 20-22. 7 
Lithidion suleatum, Gray, Cat. Cycl. p. 35, n.1; Pfr. Consp.n. 255 ; 
Pneum. Mon. p. 176. 
Hab. Yemen, Arabia (Powis, Sowerby). 
2. LITHIDION DESCISCENS. 
Shell widely umbilicate, depressed-semiglobose, slightly and closely 
spirally furrowed above, whitish; spire convex; whorls 45, mo- 
derately convex, the last one cylindrical, suddenly detlexed 
anteriorly, smooth beneath ; aperture nearly horizontal, roundish, 
sublunate, white inside ; peristome thickened, with the margins 
distant, joined by callus, basal margin reflexed, columellar mar- 
gin suddenly ascending in a curve. Height 53, greatest breadth 
10, least br. 84 mill. 
Cyclostoma desciscens, Pfr. in Proc. Z. S. 1851; Ic. n. 250, t. 35, 
f, 25, 26. 
Lithidion desciscens, Pfr. Consp. n. 256, p. 60; Pneuwm. Mon. 
pili. 
Hab. Socotora. 
3. Liruipion SouLEYETIANUM. 
Shell convex-conoidal, solid, sculptured above with six raised 
spiral ridges and with crowded transverse striz, shining, brown- 
