TROCHIDA. 315 
TROcHIscuUs, Sowb. (Norrisia, Bayle.) Shell thick, conoidal, 
orbicular, covered by an epidermis, smooth ; widely umbilicated, 
umbilicus surrounded by the callous extension of the columella; 
outer lip not thickened or sculptured within. Operculum with 
the edges of the whorls elevated, scaly. TZ. Norrisii, Sowb. 
(Ixxxi, 64). West Coast of North America. 
DILOMA, Philippi. Shell conoidal, smooth, not umbilicated ; 
whorls rather few, convex; aperture subrotund ; columellar lip 
excavated in the middle and expanded over the umbilical region, 
produced laterally to join the outer lip; outer lip thin, unarmed. 
12 sp. Pacific. 7. Hthiops, Gmel. 
PHOTINULA, H.and A. Adams. Shell orbicular, heliciform, sub- 
depressed, not umbilicated ; spire acute; whorls with revolving 
lines, smooth, polished; umbilical region covered by callus ; colu- 
mellar lip thick, outer lip not thickened, smooth within. 2 sp. 
South Seas. 7. taeniata, Wood (1xxxi, 65). 
CHRYSOSTOMA, Swainson. Shell very thick, globular-turbinated, 
polished; spire small, body-whorl large, rounded; margin of 
aperture thick, especially on the columella; umbilicus covered 
by a callosity. Isp. Indo-Pacific. 7. Nicobaricus (1xxxi, 66). 
LioTRocHUS, Fischer. Not characterized. 7. callosus, Koch. 
Marearita, Leach. 
Distr.—20 sp. Boreal, under stones and sea-weed near low- 
water. MM. helicina, Fabr. (Ixxxi, 67). 
Shell thin, globular-conical, umbilicated; whorls rounded, 
smooth; aperture rounded, pearly; lip sharp, smooth. 
BEMBIX, Watson. Shell elevated, conical, carinate, base inflated, 
umbilicated, thin, pearly, epidermis thin, membranaceous. J. 
«ola, Watson. Japan. 
MACH#ROPLAX, Friele. Shell more or less conoidal, distinctly 
umbilicated, unicolored, variously sculptured; aperture suban- 
gulated, peristome interrupted. Operculum corneous. W. striata, 
Leach (lxxxi, 68). 
SOLARIELLA, 8. Wood. With large crenated umbilicus. Fossil, 
Crag of England ; recent, one California species. M. peramabilis, 
Carp. 
MARGARITELLA, Meek and Hayden. Shell subdiscoidal or nearly 
lenticular, thin; nacre of interior bright ; umbilicus large, deep, 
and entirely without crenate margins; volutions narrow, very 
much depressed, the last one sharply angular around the per- 
iphery, and obtusely subangular around the umbilicus; aperture 
transversely rhombic; lip thin and simple; surface cancellately 
striated in the typicalspecies. Cretaceous; United States. 1. 
Alexistriata, Evans and Shumard (1xxxi, 69, 70). 
ENIDA, Adams, 1860. Whorls ornamented with spiral and 
squamous striz, sutures canaliculated, last whorl carinated at 
