a calcareous aptychus, formed of two pieces. Harpoceras, Jurassic. Oppelia, Jurassic. Lissoceras, Jurassic and Cretaceous.
Fam. 9. Amaltheidae. Shell flattened, with a prominent carina continued anteriorly into a rostrum. Amaltheus, Lias. Cardioceras, Jurassic. Schloenbachia, Cretaceous.
Fam. 10. Stephanoceratidae. Shell not carinated, but with radiating costae, which are often bifurcated, aperture often with lateral projections which contract it, aptychus formed of two pieces. Stephanoceras, Morphoceras, Pensphinctes, Peltoceras, Jurassic. Hoplites, Cretaceous. Acanthoceras, Cretaceous. Cosmoceras, Jurassic. Various more or less uncoiled forms are related to this family, viz. Scaphites, Crioceras, Cretaceous.
Order 2. Dibranchiata (= Holosiphona, Acetabulifera)
Characters.—Cephalopoda in which the inflected margins of the epipodia are fused so as to form a complete tubular siphon (fig. 24, i). The circumoral lobes of the fore-foot carry suckers disposed upon them in rows, not tentacles (see figs. 15, 24). There is a single pair of typical ctenidia (fig. 25) acting as gills (hence Dibranchiata), and a single pair of renal organs, opening by apertures right and left of the median anus (fig. 25, r) and by similar internal pores into the pericardial chamber, which consequently does not open directly to the surface as in Nautilus. The oviducts are sometimes paired right and left (Octopoda, Oigopsida), sometimes that of one side only is developed (Myopsida). The sperm-duct is always single except, according to W. Keferstein, in Eledone moschata.
A plate-like shell is developed in a closed sac formed by the mantle (figs 20, 21), except in the Octopoda, which have none, and in Spirula (fig. 17, D) and the extinct Belemnitidae, &c., which have a small chambered shell resembling that of Nautilus with or without the addition of plate-like and cylindrical accessory developments (fig. 17, A, C, fig. 19).
The pair of cephalic eyes are highly-developed vesicles with a refractive lens (fig. 33), cornea and lid-folds,—the vesicle being in the embryo, an open sac like that of Nautilus (fig. 34). Osphradia are not present, but cephalic olfactory organs are recognized. One or two pairs of large salivary glands with long ducts are present. An ink-sac formed as a diverticulum of the rectum and opening near the anus is present in all Dibranchiata (fig. 25, t), and has been detected even in the fossil Belemnitidae. Branchial hearts are developed on the two branchial afferent blood-vessels (fig. 28, vc′, vi).
|Fig. 16.—Decapodous Cephalopods.|
|A, Cheiroteuthis Veranyi, d’Orb. (from the Mediterranean).|
B, Thysanoteuthis rhombus, Troschel (from Messina).
In the Dibranchiata the shell shows various stages of degeneration, culminating in its complete disappearance in Octopus. As in other Mollusca, there is a tendency in Cephalopods for the mantle to extend over the outside of the shell from its edges, and when these secondary mantle-folds entirely cover the shell and meet or fuse together the shell is surrounded by the mantle both externally and internally, and is said to be internal, though it remains always a cuticular structure external to the epidermis. This procebs is generally accompanied by a reduction of the size of the shell in comparison with that of the body, so that the relations of the two are gradually reversed, the body outgrows its house and instead of the