34° 



MOLLUSCS. 



a ceratite [Ceratites nodosus). 



AN AMMONITE 



(Cardioceras cor datum). 



Another well-marked group is that of the Ammonoidea, represented by the 

 goniatites of the Palaeozoic, and the various types of ammonites of the Secondary 



rocks, as well as by 



the turrilites of the 



Chalk. In all these 



the edges of the 



sutures, where they 



join the shell, are more 



or less complexly 



angulated or frilled, 



the complexity being 



very great in the 



ammonites, but a 



simpler type obtaining 



in the goniatites. 

 Whereas in the two latter the shell is coiled in a flat spiral, in the turrilites it 

 forms a cone, while in the hamites and baculites of the Chalk it is either straight 

 or partially coiled. In the ceratites and ammonites (which include Ceratites, 

 Cardioceras, and many other genera) the mouth of the body-chamber of the shell 

 was closed by an operculum, which often consists of two pieces meeting in the 

 middle line, and the whole being heart-shaped. In other forms the operculum 

 was single. Mr. Cooke observes that " some authorities hold that the members of 

 this suborder belong to the Dibranchiata, on the ground that the protoconch 

 resembles that of Spirula rather than that of the Nautiloidea. Others again 

 regard the Ammonoidea as a third and distinct order of Cephalopoda. Their 

 distribution extends from the Silurian to (possibly) the early Tertiary. No trace 

 has ever been found of an ink-sac, mandible, or hooks on the arms ; and the shell 

 was undoubtedly external." 



EDGAR A. SMITH. 



