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. 



SUB-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 tbe " hyponomous sinus " ; the remainder 

 of the aperture is more or less lobate and corresponds to the external 

 parts of tbe cireumoral crown. The shell may attain to a length of two 

 metres (Endoceras). 



FAMILY 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. Saltoceras, 

 Holm ; Silurian. FAMILY 2. ACTINOCERATIDAE. Shell straight or 

 slightly curved, with a wide siphuncle contracted at the level of tbe 

 septa by rings or swellings. Genera Actinoceras, Bronn ; from tbe 

 Silurian to the Carboniferous. Discosorus, Hall ; Silurian. Huronia, 

 Stokes ; Silurian. Loxoceras, MacCoy ; from tbe Silurian to tbe Carbon- 

 iferous. FAMILY 3. ENDOCERATIDAE. Shell straight, with a wide 

 marginal sipbuncle, tbe sipbuncular necks produced into tubes which 

 fit into one another. Genera Endoceras, Hall ; shell straight ; from the 

 Silurian. FAMILY 4. GOMPHOCERATIDAE. Shell globular, straight or 

 arcuate, the aperture contracted to the shape of a T. Genera Gompho- 

 ceras, Sowerby ; Silurian. Phragmoceras, Sowerby ; Silurian. FAMILY 5. 



