274 Mr. W. King on the Tetrabranchiate Cephalopods. 



nately curves before and behind a line corresponding to the plane 

 of the centre of the plates, so that allowing this line to pass 

 through a series of curves, the edge may be said to be divided into 

 an anterior and a posterior set of lobes, which are either simple or 

 compound, according to species ; further, these lobes, throughout 

 their whole contour, are set off with numerous pointed digitations, 

 which are invariably directed backwards, that is, towards the 

 origin of the whorls. These digitations, Dr. Buckland observes, 

 may have served as holdfasts, by which the posterior part of the 

 animal's mantle could fix itself firmly, and as it were take root 

 around the bottom of the outer chamber. 



The remains of both these divisions of the Tetrabranchians are 

 common to certain of the secondary rocks. In the Silurian portion 

 of the primary period a great many of the then existing cephalo- 

 podous shells possessed plain-edge plates, and thus they agreed so 

 far with the Nautilus ; but strictly speaking, their siphonal sheath 

 cannot be said to be central, since it is often situated within and 

 at a distance more or less from either the outer or the inner 

 margin of the plates. 



These early cephalopodous shells arrest our attention by the 

 variety of shapes which they have assumed. They may be said 

 to run into every conceivable form from a close coil to a straight 

 line. The straight ones have received the name of Orthocerus ; 

 those which are curved are called Cyrthocerus ; and such as are 

 tendril-shaped or open-coiled have been termed Gyrocerus : these 

 last appear to be closely allied to some whose coils are in contact 

 with each other, and for which may be used the provisional name 

 Discus * : again, these conduct us to a shell which is close-coiled 

 when young, but afterwards it strikes off at a tangent : this is 

 Montfort's genus Lituites. Besides these, several other kinds 

 have been described and otherwise named. 



It has been observed, that in a great many of the Silurian Ce- 

 phalopods the siphonal sheath oscillated as it were between the 

 outer and the inner edge of the plates without touching either. 

 Along with these there existed others somewhat different, inas- 

 much as the edge of their plates is more or less sinuous, and their 

 siphonal sheath is placed in some on the outer, and in others on 

 the inner margin of the plates : these constitute the genera 

 Goniatites and Clymenia. 



Now it is a remarkable fact, that in whichever genus of the 

 tetrabranchiate Cephalopods we find the edge of the plates un- 

 dulated, we in general observe the siphonal sheath approximating 



* Sowerby, in the ' Mineral Conchology,' has applied Montfort's name 

 Ellipsolithes to these shells (compare generically E.funatus, tab. 32, with 

 Nautilus undosus of the ' Silurian System '), which cannot be allowed, since 

 Montfort's genus was founded on a species of Ammonites from the Chalk 

 near Rouen. 



