visceral nerve-loop, the strands of which cross one another—this being characteristic of Streptoneura (Spengel).

Fig. 28.—Carinaria mediterranea. (From Owen.)
A, The animal. B, The shell removed. C, D, Two views of the shell of Cardiopoda.
a,	Mouth and odontophore.		h,	Border of the mantle-flap.		u,	Cerebral ganglion.
b,	Cephalic tentacles.		i,	Ctenidium (gill-plume).		v,	Pleural and pedal ganglion.
c,	Eye.		m,	Stomach.		w,	Testis.
d,	The fin-like mesopodium.		n,	Intestine.		x,	Visceral ganglion.
d’,	Its sucker.		o,	Anus.		y,	Vesicula seminalis.
e,	Metapodium.		p,	Liver.		z,	Penis.
f,	Salivary glands.		t,	Aorta, springing from the ventricle.

The Heteropoda belong to the “pelagic fauna” occurring near the surface in the Mediterranean and great oceans in company with the Pteropoda, the Siphonophorous Hydrozoa, Salpae, Leptocephali, and other specially-modified transparent swimming representatives of various groups of the animal kingdom. In development they pass through the typical trochosphere and veliger stages provided with boat-like shell.

Sub-order 1.—Taenioglossa. Radula with a median tooth and three teeth on each side of it. Formula 3 : 1 : 3.

Tribe 1.—Platypoda. Normal Taenioglossa of creeping habit. The foot is flattened ventrally, at all events in its anterior part (Strombidae). Otocysts situated close to the pedal nerve-centres. Accessory organs are rarely found on the genital ducts, but occur in Paludina, Cyclostoma, Naticidae, Calyptraeidae, &c. Mandibles usually present. This is the largest group of Mollusca, including nearly sixty families, some of which are insufficiently known from the anatomical point of view.

Fam. 1.—Paludinidae. Pedal centres in the form of ganglionated cords; kidney provided with a ureter; viviparous; fluviatile. Paludina. Neothauma, from Lake Tanganyika. Tylopoma, extinct, Tertiary.

Fig. 29.—Pterotrachea mutica seen from the right side.
(After Keferstein.)
a,	Pouch for reception of the snout when retracted.
c,	Pericardium.
ph,	Pharynx.
oc,	Cephalic eye.
g,	Cerebral ganglion.
g’,	Pleuro-pedal ganglion.
pr,	Foot (mesopodium).
v,	Stomach.
i,	Intestine.
n,	So-called nucleus.
br,	Branchial plume (ctenidium).
w,	Osphradium.
mt,	Foot (metapodium).
z,	Caudal appendage.

Fam. 2.—Cyclophoridae. No ctenidium, pallial cavity transformed into a lung; aperture of shell circular; terrestrial. Pomatias, shell turriculated. Diplommatina. Hybocystis. Cyclophorus, shell umbilicated, with a short spire and horny operculum. Cyclosurus, shell uncoiled. Dermatocera, foot with a horn-shaped protuberance at its posterior end. Spiraculum.

Fam. 3.—Ampullariidae. To the left of the ctenidium a pulmonary sac, separated from it by an incomplete septum, amphibious. Ampullaria, shell dextral, coiled. Lanistes, shell sinistral, spire short or obsolete. Meladomus.

Fam. 4.—Littorinidae. Oesophageal pouches present; pedal nerve-centres concentrated; a pedal penis near the right tentacle. Littorina, shell not umbilicated, littoral habit. Lacuna, foot with two posterior appendages, marine, entirely aquatic. Cremnoconchus, entirely aerial, Indian. Risella. Tectarius.

Fam. 5.—Fossaridae. Head with two lobes in some Rhipidoglossa. Fossaria.

Fam. 6.—Purpurinidae, extinct.

Fam. 7.—Planaxidae. Shell with pointed spire; a short pallial siphon. Planaxis.

Fam. 8.—Cyclostomatidae. Pallial cavity transformed into a lung; pedal centres concentrated; a deep pedal groove. Cyclostoma, shell turbinated, operculum calcareous, British. Omphalotropis.

Fam. 9.—Aciculidae. Pallial cavity transformed into a lung; operculum horny; shell narrow and elongated. Acicula.

Fam. 10.—Valvatidae. Ctenidium bipectinate, free; hermaphrodite; fluviatile. Valvata, British.

Fam. 11.—Rissoidae. Epipodial filaments present; one or two pallial tentacles. Rissoa. Rissoina. Stiva.

Fam. 12.—Litiopidae. An epipodium bearing three pairs of tentacles and an operculigerous lobe with two appendages; inhabitants of the Sargasso weed. Litiopa.

Fam. 13.—Adeorbiidae. Mantle with two posterior appendages; ctenidium large and capable of protrusion from pallial cavity. Adeorbis, British.

Fam. 14.—Jeffreysiidae. Head with two long labial palps; shell ovoid; operculum horny, semicircular, carinated. Jeffreysia.

Fam. 15.—Homalogyridae. Shell flattened; no cephalic tentacles. Homalogyra, British. Ammoniceras.

Fam. 16.—Skeneidae. Shell depressed, with rounded aperture; cephalic tentacles long. Skenea, British.

Fam. 17.—Choristidae. Shell spiral; four cephalic tentacles; eyes absent; two pedal appendages. Choristes.

Fam. 18.—Assimineidae. Eyes at free extremities of tentacles. Assiminea, estuarine, British.

Fam. 19.—Truncatellidae. Snout very long, bilobed; foot short. Truncatella.

Fig. 30.—Valvata cristata, Müll.
o,	Mouth.
op,	Operculum.
br,	Ctenidium (branchial plume).
x,	Filiform appendage (? rudimentary ctenidium).
The freely projecting ctenidium of typical form not having its axis fused to the roof of the branchial chamber is the notable character of this genus.

Fam. 20.—Hydrobiidae. Shell with prominent spire; penis distant from right tentacle, generally appendiculated; brackish water or fluviatile. Hydrobia, British. Baikalia, from Lake Baikal. Pomatiopsis. Bithynella. Lithoglyphus. Spekia, viviparous, from Lake Tanganyika. Tanganyicia. Limnotrochus, from Lake Tanganyika. Chytra. Littorinida. Bithynia, British, fluviatile. Stenothyra.

Fam. 21.—Melaniidae. Spire of shell somewhat elongated; mantle-border fringed; viviparous; fluviatile. Melania. Faunus. Paludomus. Melanopsis. Nassopsis. Bythoceras, from Lake Tanganyika.

Fam. 22.—Typhobiidae. Foot wide; shell turriculated, with carinated whorls, the carinae tuberculated or spiny. Typhobia. Bathanalia, from Lake Tanganyika.

Fam. 23.—Pleuroceridae. Like Melaniidae, but mantle-border not fringed and reproduction oviparous. Pleurocera. Anculotus.

Fam. 24.—Pseudomelaniidae. All extinct.

Fam. 25.—Subulitidae. All extinct.

Fam. 26.—Nerineidae. All extinct.

Fam. 27.—Cerithiidae. Shell with numerous tuberculated whorls; aperture canaliculated anteriorly; short pallial siphon. Cerithium. Bittium. Potamides. Triforis. Laeocochlis. Cerithiopsis.

Fam. 28.—Modulidae. Shell with short spire; no siphon. Modulus.