In Pleurotomaria we have the case of a genus long supposed to be extinct. More than 1100 fossil species have been described, and within the last 38 years about 20 specimens, belonging to 5 species, have been discovered in a living state.
Fig. 269.—Pleurotomaria adansoniana Cr. and F., Tobago. × ½.
Fam. 4. Bellerophontidae.—Shell nautiloid, spire generally concealed, aperture large, sinus or perforations central (Fig. [179], p. 266). Ordovician—Trias. Genera: Bellerophon, Trematonotus, Cyrtolites.
Section II. Azygobranchiata.—One ctenidium (the left) present.
Fam. 1. Cocculinidae.—A single cervical ctenidium, foot broad, no eyes, shell patelliform, with caducous spire. Single genus, Cocculina. Deep water.
Fam. 2. Stomatellidae.—A single (left) ctenidium, front third free, shell nacreous, spiral or patelliform, depressed, last whorl large. Jurassic——. Genera: Stomatella (subg. Synaptocochlea, Niphonia), shell depressed, spirally ribbed, spire short, operculum present; Phaneta, fluviatile only, shell trochiform, imperforate, last whorl keeled, sinuate in front; Stomatia, spire short, surface tubercled or keeled, no operculum; Gena, shell haliotis-shaped, surface smooth, aperture very large; Broderipia, shell patelliform, spiral apex often lost.
Fam. 3. Cyclostrematidae.—Tentacles ciliated, thread-like, snout bilobed, foot truncated in front, angles produced into a filament, shell depressed, umbilicated, not nacreous. Eocene——. Principal genera: Cyclostrema, Teinostoma, Vitrinella.
Fam. 4. Liotiidae.—Epipodial line with a lobe behind each eye-peduncle, shell solid, trochiform, longitudinally ribbed or trellised, aperture round, operculum multispiral, hispid, corneous, with a calcareous layer. Silurian——. Principal genera: Liotia, Craspedostoma (Silurian), Crossostoma (Jurassic).
