Suborder 1. Decapoda.—Four pairs of ordinary non-retractile arms which are shorter than the body, and one pair of tentacular arms, situated between the third and fourth normal arms on each side and retractile within special pouches. Suckers pedunculated and provided with horny rings, on the tentacular arms confined usually to the distal extremities. Usually a well-developed internal shell, and lateral fins on the edges of the body. Heart in a coelomic cavity; nidamentary glands usually present.
Tribe 1. Oigopsida.—A wide aperture in the cornea. Two oviducts in the female. In fossil genera and Spirula, shell has a multilocular phragmacone with a siphuncle; initial chamber globular and larger than the second chamber. The most ancient forms characterized by the small size of the rostrum and proöstracum, and large size of the phragmacone. In the living genera, except Spirula, the shell is a chitinous gladius.
Fam. 1. Belemnoteuthidae. Extinct; shell with well-developed phragmacone, and rostrum merely a calcareous envelope; siphuncular necks directed backwards as in Nautiloidea; ten equal arms provided with hooks. Phragmoteuthis, Trias. Belemnoteuthis, Jurassic and Cretaceous. Acanthoteuthis, Jurassic.
Fam. 2. Aulacoceratidae. Extinct; phragmacone with widely separated septa; rostrum well developed and claviform. Aulacoceras, Trias. Atractites, Trias and Jurassic. Xiphoteuthis, Lias.
Fam. 3. Belemnitidae. Extinct; phragmacone short with ventral siphuncle, prolonged dorsally into long proöstracum; rostrum large and cylindrical. Belemnites, 350 species from Jurassic and Cretaceous. Diploconus, Upper Jurassic.
Fam. 4. Belopteridae. Extinct; rostrum and phragmacone well developed, phragmacone often curved; initial chamber small. Beloptera, Eocene. Bayanoteuthis, Eocene. Spirulirostra, Miocene.
Fam. 5. Spirulidae. Dorsal and ventral sides of posterior extremity of shell uncovered by mantle; no rostrum or proöstracum; shell calcareous, coiled endogastrically and sipnunculated; fins posterior. Spirula, three living species known, abyssal.
Fam. 6. Ommatostrephidae. Shell internal and chitinous, ending aborally in a little narrow cone; tentacular arms short and thick; suckers with denticulate rings. Ommatostrephes, fins aboral, simple and rhomboidal, British. Ctenopteryx, fins pectinate, as long as the body; Bathyteuthis, fins terminal, rudimentary; tentacular arms, filiform; abyssal. Rhynchoteuthis, tentacular arms united to form a beak-shaped appendage. Symplectoteuthis. Tracheloteuthis. Doridicus. Architeuthis; this is the largest of Cephalopoda, reaching 60 ft. in length including arms.
Fam. 7. Thysanoteuthidae. Arms enlarged, bearing two rows of suckers and filaments; fins triangular, extending whole length of body. Thysanoteuthis, Mediterranean.
Fam. 8. Onychoteuthidae. Fins terminal; tentacular arms long; suckers with hooks. Onychoteuthis, hook-bearing suckers on tentacular arms only. Enoploteuthis, hook-bearing suckers on all the arms. Veranya, body very short, tentacular arms atrophied in the adult, Mediterranean. Chaunoteuthis, body elongated, tentacular arms atrophied. Pterygioteuthis. Ancistroteuthis. Abralia. Teleoteuthis. Lepidoteuthis.
Fam. 9. Gonatidae. Body elongated; fins terminal; radula with only two lateral teeth. Gonatus.
Fam. 10. Cheiroteuthidae. Tentacular arms long, not retractile; resisting apparatus well developed. Cheiroteuthis, suckers along the whole length of the tentacular arms. Doratopsis, body very long and slender with aboral spine, dorsal arms very short. Histioteuthis, six dorsal arms united by membrane, photogenous organs present. Histiopsis, membrane of dorsal arms only half-way up the arms, photogenous organs present. Calliteuthis, no brachial membrane, photogenous organs present. Grimalditeuthis, two fins on each bide, no tentacular arms.
Fam. 11. Cranchiidae. Eight normal arms, very short; eyes prominent; fins small and terminal. Cranchia, body short, purse-shaped, normal arms short, fins entirely aboral. Loligopsis, body elongated, conical, tentacular arms slender. Leachia, tentacular arms absent, funnel without a valve. Taonius, body elongated, normal arms, rather short, eyes pedunculated.
Tribe 2. Myopsida.—No aperture in the cornea. Left oviduct only developed in female. Internal shell without a distinct phragmacone, calcified or simply chitinous.
Fam. 1. Sepiidae. Body wide and flat; fins narrow, extending the whole length of the body; shell calcareous and laminated. Belosepia, a rudiment of rostrum and phragmacone present in shell, Eocene. Sepia, shell with a rostrum, British. Sepiella, shell without a rostrum.