Section Ii. Heteropoda.--Foot Fin-Shaped, Not Flat.

The Heteropoda are free-swimming Mollusca, being, like the Pteropoda, Gasteropoda modified to suit their pelagic environment. Their nervous system is streptoneurous, and they are therefore probably derived from the Prosobranchiata, but they are highly specialised forms. Pelseneer considers them far more widely removed from the Streptoneura than the Pteropoda are from the Euthyneura. They swim on the surface “upside down,” i.e. with the ventral side uppermost.

The tissues and shell are transparent, permitting observation of the internal organs. In the Pterotrachaeidae the foot takes the form of a fan-shaped disc, usually furnished with a sucker. The body is compressed at the posterior end, often with a ventral “fin.” In Atlanta the foot consists of three very distinct parts: a propodium, a mesopodium, on which is a small sucker, and a metapodium, which carries the operculum. The branchiae are carried on the visceral sac, and are free in Pterotrachaea, slightly protected by the shell in Carinaria, and entirely covered in Atlanta; absent altogether in Firoloida.

The head carries two tentacles (except in Pterotrachaea), with large, highly organised eyes on short lobes at their outer base. The alimentary tract consists of a long protrusible proboscis, with a taenioglossate radula (Fig. 132, p. 227), a long oesophagus, and a slightly flexured intestine. In Atlanta the visceral sac is spiral and protected by a spiral planorbiform shell; in Carinaria the visceral sac is small, conical, protected by a very thin capuliform shell. There is no shell in Pterotrachaea or Firoloida.

The Heteropoda are dioecious. In the male there is a flagellum behind the penis, which is near the middle of the right side. Pterotrachaea lays long chains of granular eggs, and has been noticed to produce a metre’s length in a day. The eggs of Atlanta are isolated. The embryo has a deeply bilobed velum.

FAM. 1. Pterotrachaeidae.--Body long, with a caudal “fin;” branchiae dorsal, free or partly protected by a shell; foot consisting of a muscular disc, with or without a sucker.

Pterotrachaea proper has no mantle, shell, or tentacles. The branchiae are disposed round the visceral sac, at the upper part of which is the anus. In Firoloida the body is abruptly truncated behind, with a long filiform segmented caudal appendage; visceral sac at the posterior end: fin-sucker present or absent in both male and female. Cardiapoda resembles Carinaria, but the visceral sac is more posterior and is only slightly protected by a very small spiral shell. Carinaria (Fig. 279) has a rugose translucent skin, visceral sac sub-median, apparently pedunculated, covered by a capuliform shell. The larval shell, which persists in the adult, is helicoid.

FAM. 2. Atlantidae.--Shell spiral, operculate, covering the animal. Branchiae in a dorsal cavity of the mantle; foot trilobed, with a small sucker on the mesopodium.

The shell of Atlanta is discoidal and sharply keeled, while that of Oxygyrus is nautiloid, with the spire concealed, no keel, aperture dilated.

(c) GYMNOGLOSSA.--Radula and jaws absent; proboscis prominent, sexes probably separate, penis present. The section is probably artificial and unnecessary, the families composing it being, in all probability, Taenioglossa which have lost their radula in consequence of changed conditions of life (pp. 79, 225).

FAM. 1. Eulimidae.--Proboscis very long, retractile, mantle forming a siphonal fold; shell small, long, subulate, polished; suture shallow, aperture continuous, operculum present or absent. Animal often parasitic, sucking the juices of its host by its long proboscis. Trias----. Genera: Eulima (subg., Subularia, Arcuella, Apicalia, Mucronalia, Stiliferina, and others), Stilifer, Scalenostoma, Niso, and Hoplopteron.

FAM. 2. Pyramidellidae.--Tentacles auriform, proboscis as in Eulimidae, a prominent mentum or flap under the buccal orifice; shell usually small, conical; suture shallow, apical whorls (the embryonic shell) sinistral (p. 250), operculum corneous, paucispiral; nucleus excentrical. Trias----. Genera: Pyramidella (subg., Syrnola, Otopleura, Chrysallida, Mumiola), Odostomia, Eulimella, Murchisoniella, Turbonilla (subg., Dunkeria and Cingulina).

(d) RACHIGLOSSA (p. 220).--Proboscis long, retractile; siphon distinct, radula without uncini, sometimes without laterals; teeth strongly cusped; shell generally wholly external.

FAM. 1. Muricidae.--Eyes sessile at the outer base of the tentacles, penis large, behind the right tentacle, radula within the retractile proboscis, central tooth (Fig. 119, p. 220) with at least three strong cusps, laterals plain; shell solid, more or less tuberculate, spiny and varicose, anterior canal varying from a mere notch to a long channel. Cretaceous----. Principal genera: (i.) Muricinae, nucleus of operculum sub-terminal; Trophon, Typhis, Murex (with many subdivisions), Ocinebra (including Cerastoma, Vitularia, and Hadriania), Urosalpinx, Eupleura, Pseudomurea. (ii.) Purpurinae, nucleus of operculum lateral; Rapana (including Latiaxis), Purpura (with subg., Cuma, Iopas, Vexilla, and Pinaxia), Monoceros (including Chorus), Purpuroidea (Secondary strata), Pentadactylus, Sistrum, Concholepas.

FAM. 2. Coralliophilidae.--Animal living in Madrepores, resembling Purpura, radula absent; shell variously shaped, often deformed or tubular, operculum that of Purpura, if present. Miocene----. Principal genera: Rhizochilus, Coralliophila, Leptoconchus, Magilus (Fig. 29, p. 75), Rapa.

FAM. 3. Columbellidae.--(Radula, Fig. 123, p. 222.) Shell small, solid, fusiform, aperture narrow, canal short, outer lip thickened. Miocene----. Single genus, Columbella (subg., Nitidella, Anachis, Meta, Strombina, Atilia, Conidea, Amphissa, Mitrella, and others).

FAM. 4. Nassidae.--Foot long and broad, often with terminal appendages; siphon long, eyes on outer base of tentacles, central tooth of radula arched, multicuspid, lateral strongly bicuspid, with small denticles between the cusps; shell rather small, buccinoid, columella more or less callous, outer lip thickened, often toothed; operculum corneous, edges often toothed. Miocene----. Principal genera: Nassa (with many sections), Amycla, Desmoulea, Cyclonassa, Canidia (subg., Clea and Nassodonta), Dorsanum, Bullia (= Buccinanops, Fig. 62, p. 185), Truncaria.

FAM. 5. Buccinidae.--Siphon rather long, eyes at outer base of tentacles; central tooth of radula with 5 to 7 cusps, laterals bicuspid or tricuspid (Fig. 118, p. 220); shell more or less fusiform, thick, covered with a periostracum, canal of varying length, outer lip simple or thickened; operculum corneous, nucleus variable in position. Cretaceous----. Principal genera: Group i. Chrysodomus (with sections Neptunea, Volutopsis, Pyrolofusus, Jumala), subg., Sipho; Siphonalia (subg., Kelletia). Group ii. Liomesus (= Buccinopsis). Group iii. Buccinum (Fig. 1 B, p. 6; subg., Volutharpa, Neobuccinum). Group iv. Cominella, Tritonidea, Pisania, Euthria; Anura (Miocene), Genea (Pliocene), Metula, Engina. Group v. Phos, Hindsia. Group vi. Dipsaccus (= Eburna), Macron. Group vii. Pseudoliva.

FAM. 6. Turbinellidae.--Central tooth of radula tricuspid, median cusp strong, lateral bicuspid, cusps unequal (Fig. 117, p. 220); shell fusiform or pear-shaped, heavy, canal often long, operculum corneous, claw-shaped, nucleus terminal. Miocene----. Principal genera: Turbinella, Cynodonta, Tudicla (subg., Streptosiphon); Piropsis (Cretaceous), Perissolax (Cretaceous), Strepsidura (Eocene, subg., Whitneya), Melapium, Fulgur (= Busycon, Fig. 150, p. 249, including Sycotypus), Melongena (subg., Pugilina, Myristica); Liostoma (Eocene), Hemifusus (subg., Megalatractus), Ptychatractus, Meyeria.

FAM. 7. Fasciolariidae.--Eyes at the outer base of the tentacles (radula, Fig. 121, p. 221); shell fusiform, spire long, canal often very long, columella often with a fold at the base; operculum corneous, nucleus terminal. Cretaceous----. Principal genera: Fusus (including Sinistralia, Aptyxis, Troschelia), with subg., Serrifusus (Cretaceous), Clavella (subg. Thersites), Fasciolaria, Latirus (subg. Polygona, Peristernia, Leucozonia, Lagena; Mazzalina (Eocene), Chascax).

FAM. 8. Mitridae.--Siphon rather long, with anterior appendages, eyes on the side of the tentacles, proboscis very long; radula variable, laterals sometimes lost (Fig. 120, p. 221); shell fusiform, solid, spire more or less pointed, columella with several prominent folds, the posterior the largest, aperture rather narrow, no operculum. Cretaceous----. Principal genera: Mitra (with many sections), subg., Strigatella, Mitreola, Mutyca, Dibaphus; Plochelaea (Tertiary), Thala; Turricula (with several sections), Cylindromitra, and Imbricaria.

FAM. 9. Volutidae.--Foot broad in front, head laterally dilated into lobes, on which are placed the sessile eyes; siphon prominent, with appendages at the base (radula, Fig. 122, p. 221); shell thick, often shining, fusiform, globular or cylindrical, columella projecting anteriorly, with several folds, the anterior of which is the largest, aperture notched, canal not produced, operculum generally absent. Cretaceous----. Principal genera: Cryptochorda (Eocene), Zidona, Provocator, Guivillea, Yetus (= Cymbium), Voluta (with many sections), Volutolithes (chiefly Eocene), Volutolyria, Lyria, Enaeta, Volutomitra.

FAM. 10. Marginellidae.--Foot broad, siphon without appendages, mantle largely reflected over the shell; radula without laterals, central tooth comb-like, cusps rather blunt; shell oval or conoidal, polished, aperture narrow, outer lip thickened, columella with many folds; no operculum. Eocene----. Principal genera: Marginella, with many sections and so-called sub-genera; Persicula, Pachybathron (?), Cystiscus, Microvoluta.

FAM. 11. Harpidae.--Foot large, with a transverse groove, separating off a semi-lunar propodium; mantle partly reflected over the shell; shell ventricose, polished; spire short, strongly longitudinally ribbed, ribs prolonged over the suture, columella callous; no operculum. Eocene----. Single genus, Harpa (subg., Silia).

FAM. 12. Olividae.--Propodium semi-lunar, with a longitudinal groove above, mesopodium reflected laterally over the shell; central tooth of radula tricuspid on a very broad base, lateral simple, hooked; shell sub-cylindrical or fusiform, polished; aperture narrow, operculum present or absent. Cretaceous----. Principal genera: Oliva (Figs. 283, and 98, p. 199), Olivancillaria (including Lintricula and Agaronia), Olivella, Ancilla (subg., Ancillina).

(e) TOXOGLOSSA (p. 218).--Radula with normal formula 1·0·1, teeth large; oesophagus with a large poison gland; animal carnivorous, exclusively marine.

FAM. 1. Terebridae.--Eyes at the end of the tentacles, shell subulate, many whorled, operculum with terminal nucleus. Eocene----. Single genus, Terebra, with several sections.

FAM. 2. Conidae.--Eyes on outer side of tentacles, siphon prominent; shell conical or fusiform, aperture narrow. Cretaceous----. Principal genera: Conus, shell solid, spire short, aperture narrow, straight, internal partitions partly absorbed; Conorbis, Genotia (with several sections, chiefly Tertiary), Pusionella, Columbarium, Clavatula, Surcula, Pleurotoma; Borsonia (Eocene), Drillia (subg., Spirotropis), Bela, Mangilia (including Daphnella, Clathurella, and others), Halia.

FAM. 3. Cancellariidae.--Proboscis short, usually no radula, shell oval, columella strongly plicate; no operculum. Cretaceous----. Single genus, Cancellaria (subg., Merica, Trigonostoma, Admete).