NERITID. 293 
developed, usually soldered by its extreme edge to the last 
whorl, and forming over the umbilicus an arch, having the 
opening larger than the aperture of the shell. S. Philippiana, 
C. B. Ad. (Ixxvii, 45, 46). 
PROSERPINA, Guilding. 
Syn.—Odontostoma, d’Orb. 
Distr.—i sp. West Indies, Northern So. America. P. depressa, 
d’Orb. (Ixxvii, 47). 
Shell orbicular-depressed, polished, covered at the base by a 
shining callus; aperture semioval, lip sharp; columella and 
parietal wall with dentiform lamellae. No operculum. 
PROSERPINELLA, Bland, 1865. Columellar fold absent ; having a 
single parietal tooth. P. Berendti, Bld. Mexico. 
oYANE, H. Ad.,1870. Columella truncate below. P. Blandiana, 
Ad. Peru. 
CeREs, Gray. 
Distr.—2 sp. Mexico. C. eolina, Duclos (Ixxvii, 49). Shell 
heliciniform, carinated at the periphery, upper surface rugose, 
epidermis thin, callous at the base; lip sharp, with spiral laminze 
within the aperture ; columella with dentiform lamine. 
OrpeR SCUTIBRANCHIATA. 
Branchiz pectinated, placed in a cavity in the upper part of 
the neck, or at the inferior edge of the mantle, around the foot. 
Dicecious. 
Shell usually spiral (globular or pyramidal) or conical, holos- 
tomate. 
Suborder popoprHanMA. Shell spiral. Operculum usually 
present and paucispiral or multispiral. Eyes on peduncles, 
separate from the tentacles. 
Suborder EpRIopTHALMA. Shell conical, not spiral; no oper- 
culum ; eyes sessile. 
SuporpeR PODOPTHALMA. 
Famity NERITID A. 
Shell thick, semiglobose, porcellanous; spire very small; cavity 
simple, from the absorption of the internal portions of the whorls ; 
aperture semilunate; columellar side expanded and flattened ; 
outer lip acute. Operculum shelly, subspiral, articulated. 
At each end of the columella there is an oblong muscular 
impression, conneeted on the outer side by a ridge, on which 
the operculum rests; within this ridge the inner layers of the 
shell are absorbed. 
Animal with a broad, short muzzle, and long slender tentacles; 
eyes on prominent pedicels, at the outer bases of the tentacles ; 
