Many theories have been proposed in explanation of these structures. According to Sir Richard Owen, the aptychus is an operculum developed in a part of the body corresponding to the hood of Nautilus. E. Ray Lankester suggested that the double plate was borne on the surface of the nidamental gland, with the form and sculpturing of which in Nautilus it closely agrees. On this view the aptychus would occur only in females. The most recent view is that these structures could not have been opercula because of their constant position inside the body-chamber, and that they were not external secretions at all, but a calcified internal cartilage situated at the base of the funnel.
Classification of Tetrabranchiata.—Cephalopoda in which the mantle is entirely enclosed by a multilocular siphunculated shell, which may or may not be coiled. Only the last compartment of the shell occupied by the body of the animal. Numerous pedal tentacles around the mouth, which are retractile within sheaths. Halves of the funnel not united. Two pairs of ctenidia, and two pairs of renal tubes without reno-pericardial apertures. Pericardium opens directly to exterior. Cephalic cartilage wholly ventral. Optic vesicles with apertures, without crystalline lens.
Sub-order 1. Nautiloidea.—Initial chamber not inflated, with dorso-ventral cicatrix at extremity.
Fam. 1. Orthoceratidae. Shell straight or slightly curved, with a simple aperture, large terminal chamber and cylindrical siphuncle. Orthoceras, Silurian to Trias. Baltoceras, Silurian.
Fam. 2. Actinoceratidae. Shell straight or slightly curved, with wide siphuncle contracted at level of septa. Actinoceras, Silurian to Carboniferous. Discosorus, Silurian. Huronia, Silurian. Loxoceras, Silurian to Carboniferous.
Fam. 3. Endoceratidae. Shell straight, with wide margina siphuncle, necks produced into tubes fitting into one another. Endoceras, Silurian.
Fam. 4. Gomphoceratidae. Shell globular, straight or arcuate, aperture contracted. Gomphoceras, Silurian. Phragmoceras, Silurian.
Fam. 5. Ascoceratidae. Shell straight, ampulliform, summit truncate, terminal chamber extending nearly whole length of shell ventrally. Ascoceras, Silurian. Glossoceras, Silurian.
Fam. 6. Poterioceratidae. Shell straight or curved, fusiform, aperture simple, siphuncle contracted at septa. Poterioceras, Silurian to Carboniferous. Streptoceras, Silurian.
Fam. 7. Cyrtoceratidae. Shell slightly curved, aperture simple, siphuncle wide, septa approximated. Cyrtoceras, Devonian.