II. 



Cephalopod Beginnings. 



''PHE chief object of this article is to draw attention to some obser- 

 1 vations recently made in North America by Mr. J. M. Clarke 

 of Albany, and published in the American Geologist (6, 7, 8) ; to make 

 in passing such criticisms as may seem desirable ; and to consider 

 the relation of the new facts to the general question of Cephalopod 

 evolution. 



First, however, for the sake of those whose studies have not lain 

 specially among the Cephalopoda, it may be advisable to explain a 

 few of the terms employed. 



The shell of a typical cephalopod, such as Nautilus, Ammonites, or 

 Ovthocevas, consists of a very elongate cone, which may be straight 

 or curved or coiled, but which is always divided interiorly by 

 transverse partitions, or ' septa,' into a number of chambers, or 

 'loculi.' Broadly speaking, these loculi gradually increase in size 

 from the apex of the cone towards its aperture, not merely because 

 the width of the cone increases, but because the septa are placed 

 at a gradually increasing distance from one another. The shell, as 

 thus described, may be called the ' conch ' or the * phragmacone,' 

 the latter word meaning nothing more than septate cone. The 

 soft body of the animal is lodged in the wide end of the shell, in 

 front of the last-formed septum ; and the cup-like space in which it 

 lies is called the ' body-chamber.' That part of the body which 

 abuts on the last-formed septum and which fills the bottom or hinder 

 part of the body-chamber, contains the chief viscera of the animal, 

 especially the generative glands, and is known as the ' visceral 

 hump.' This hump is covered with a thin, glandular skin, which 

 secretes membranes of an organic substance called ' conchiolin ' ; 

 as these membranes become impregnated with lime (calcified), the 

 septa and the inner lining of the shell-wall are thus formed. The 

 formation of septa in the cephalopod shell is precisely parallel to the 

 formation of septa in other molluscs and in other groups of animals, 

 such as the tabulate corals; that is to say, the soft body with its 

 posterior secreting surface moves forward at intervals in its cone or 

 tube. As to the causes of this forward movement there has been 

 much speculation, into which it is now unnecessary to enter. One 

 thing, however, seems fairly clear, namely, that the formation of 



