Fam. 2. Lepetidae.—No ctenidia or accessory branchiae, animal generally blind. Pliocene——. Principal genera: Lepeta; Propilidium, apex with internal septum; Lepetella.
Fam. 3. Patellidae.—No ctenidia, the osphradial patch at the base of each alone surviving, a circlet of secondary branchiae between the mantle and sides of the foot. Ordovician——. (i.) Patellinae.—Three lateral teeth on each side, two of them anterior. Principal genera: Patella, branchial circlet complete; chief sections Patella proper, Scutellastra, Ancistromesus (A. mexicana Brod., measures 8–14 in. long); Helcion, branchial circlet interrupted in front; Tryblidium (Ordovician).—(ii.) Nacellinae.—Two developed laterals on each side, one anterior. Genera: Nacella, branchial circlet complete; Helcioniscus, branchial circlet interrupted in front.
(b) Rhipidoglossa (p. [225]).—Ventricle of the heart traversed by the rectum (except in Helicinidae), one or two ctenidia; jaw in two pieces, radula long, marginals multiplied, rows curved.
Of all the Gasteropoda, this section of the Diotocardia approach nearest to the Pelecypoda, particularly in the least specialised forms. The auricle, the branchiae, and the kidneys are in many cases paired, and more or less symmetrical. The ventricle is generally traversed by the rectum, there is a long labial commissure between the cerebral ganglia, special copulative organs are usually absent, while the shell is often nacreous, like those of Pelecypoda of a primitive type.
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.
Fig. 268.—Scutus australis Lam., Australia: m, m, mantle; sh, shell, × ⅔.
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.