Chapter XX: Section I: ZYGOBRANCHIATA.--Two ctenidia, shell with apical or marginal

slit or holes, corresponding to an anal tube in the mantle (p. 265).

FAM. 1. _Fissurellidae._--Two symmetrical ctenidia and kidneys, visceral mass conical, shell conical, elevated or depressed, with a single anterior or apical slit or impression; no operculum. Jurassic----. (i.) _Fissurellinae_. Shell wholly external, apex entirely removed by perforation, apical callus not truncated posteriorly; central tooth narrow. Genera: _Fissurella_ (Figs. 171, p. 261; 178, p. 265), _Fissuridea_, _Clypidella_. (ii.) _Fissurellidinae_. Shell partly internal, otherwise as in (i.); central tooth broad, mantle more or less reflected over the shell, apical hole very wide. Genera: _Fissurellidaea_, _Pupillaea_, _Lucapina_, _Megatebennus_, _Macroschisma_, _Lucapinella_. (iii.) _Emarginulinae_. Shell usually wholly external, apex usually not removed by perforation, sometimes with internal septum, anal tube in a narrow slit or sinus. Genera: _Glyphis_, externals of _Fissurella_, but holecallus truncated behind; _Puncturella_ (sub-genera _Cranopsis_ and _Fissurisepta_), slit just anterior to the apex, a small internal septum; _Zeidora_, large internal septum as in _Crepidula_: _Emarginula_, shell elevated, slit very narrow, on the anterior margin (in subg. _Rimula_, it is between the apex and the margin), radula bilaterally asymmetrical; _Subemarginula_, margin indented by a shallow groove; _Scutus_ (= _Parmophorus_) shell oblong, depressed, nicked in front, largely covered by the mantle.

FAM. 2. _Haliotidae_.--Right ctenidium the smaller, epipodial line broad, profusely lobed; shell rather flattened, spire short, last whorl very large, with a row of perforations on the left side, which become successively obliterated; through these holes, the posterior of which is anal, pass tentacular appendages of the mantle; no operculum. Cretaceous----. Single genus, _Haliotis_; principal sub-genera _Padollus_, _Teinotis_.

FAM. 3. _Pleurotomariidae_.--Central tooth single, narrow, about 26 laterals, 60 to 70 uncini. Shell generally variously trochiform, nacreous, operculate, with a rather broad marginal sinus in the last whorl; as this sinus closes up it forms an “anal fasciole” or “sinus band.” Cambrian----. Principal genera: _Scissurella_, epipodial line with several long ciliated appendages at each side, shell very small, slit open, sinus band extending nearly to apex; _Schismope_, anal slit closed in the adult into an oblong perforation; _Murchisonia_ (Palaeozoic only), shell long, turreted, whorls angulate or keeled with a sinus band; _Odontomaria_ (Palaeozoic only), shell tubular, curved; _Polytremaria_ (Carboniferous), shell turbinate, slit a series of small holes connected by a passage; _Trochotoma_, shell trochiform, perforation consisting of two narrow holes united by a slit; _Pleurotomaria_, branchiae almost symmetrical, radula as above, shell variously spiral.

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.

FAM. 4. _Bellerophontidae_.--Shell nautiloid, spire generally concealed, aperture large, sinus or perforations central (Fig. 179, p. 266). Ordovician--Trias. Genera: _Bellerophon_, _Trematonotus_, _Cyrtolites_.