332 THE CEPHALOPODA 
ORDER 1. Tetrabranchia, Owen. 
In these Cephalopoda the whole of the visceral mass is protected 
by an external, multilocular, siphunculated shell, which may or may 
not be coiled; only the last compartment of the shell is occupied 
by the body of the animal. The head bears numerous appendages 
in the form of pedal tentacles, which are retractile within sheaths 
(Fig. 293, te). The funnel is formed of two separate moieties. There 
are four branchiae, and four kidneys without reno-pericardial orifices. 
The pericardium opens directly to the exterior. The cephalic 
cartilage is wholly situated on the ventral side of the oesophagus 
(Fig. 270, h) and only supports the ventral part of the nervous 
centres. The eyes are open and have no crystalline lens (Fig. 6, A). 
The Tetrabranchia comprise two sub-orders, the Nautiloidea and 
the Ammonitoidea. 
Sus-ORDER 1. NAUTILOIDEA. 
This group is distinguished from the Ammonitoidea by the initial 
chamber, which is in the form of an obtuse cone bearing on its summit a 
“ cicatrix,” elongated dorso-ventrally and situated opposite the extremity 
of the blind end of the siphuncle: it is probable that the siphuncle passed 
through this cicatrix on emerging from a true initial chamber or proto- 
conch, which may have been uncalcified or caducous. The sub-order 
comprises nearly 2500 fossil species, but only a few living species of the 
genus Nautilus, In certain fossil forms the aperture of the shell may be 
contracted to such an extent that the animal was probably able to protrude 
only the appendages of the circumoral crown, but not its head. These 
contracted apertures are said to be “composite” when they have lobes of 
different form, as in Gomphoceras, Phragmoceras, etc. In these apertures 
the ventral part, corresponding to the funnel, is separated from the rest 
by a constriction, and constitutes the “ hyponomous sinus” ; the remainder 
of the aperture is more or less lobate and corresponds to the external 
parts of the cireumoral crown, The shell may attain to a length of two 
metres (Hndoceras). 
Famity 1. ORTHOCERATIDAE. Shell straight or slightly curved, with 
a simple aperture, a large terminal chamber, and a cylindrical siphuncle. 
Genera—Orthoceras, Breyn ; from the Silurian to the Trias. Baltoceras, 
Holm; Silurian. Faminy 2. AcTINoceRATIDAE. Shell straight or 
slightly curved, with a wide siphuncle contracted at the level of the 
septa by rings or swellings. Genera—Actinoceras, Bronn; from the 
Silurian to the Carboniferous. D¢scosorus, Hall; Silurian. Huwronia, 
Stokes ; Silurian. Lozoceras, MacCoy ; from the Silurian to the Carbon- 
iferous. Famity 3. EnpoceRATIDAE. Shell straight, with a wide 
marginal siphuncle, the siphuncular necks produced into tubes which 
fit into one another. Genera—Endoceras, Hall ; shell straight ; from the 
Silurian. Faminy 4. GoMPHOCERATIDAE. Shell globular, straight or 
arcuate, the aperture contracted to the shape of a T. Genera—Gompho- 
ceras, Sowerby ; Silurian. Phragmoceras, Sowerby ; Silurian, Famity 5. 
