340 
MOLLUSCS. 
AN AMMONITE 
(Cardioceras cordatum). 
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 
A ceratite (Cemtites nodosus). 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 Sjnnda 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. 
