b 
28 
this on the right hand flgure of the diagram representing a Triassic 
form known as a Ceratite. You must distinguish between the 
actual ribs and projections of the shell wall itself, and these 
sinuosities which are the edges of the septa. In this example you 
notice that there is a simple curve directed forwards, towards the 
mouth of the shell—this is a ‘“‘ saddle’; and then there is an 
elevation between the saddles directed backwards, known as a 
‘‘lobe,”’ which is toothed or denticulated. That is the distinguish- 
ing mark of a Ceratite. In the true Ammonite both lobes and 
saddles may be irregular in outline. On the table are two casts of 
chambers of an Ammonite, by looking at these you will get an idea 
of their very irregular shape. You will notice too, on comparing 
diagrams and specimens, that the outer chamber or habitation of 
the Ammonite differed much in shape from that of the Nautilus. 
It was much more elongated, and not so expanded in width. 
The earliest known forms of the family are found in the Lower 
Silurian, but in this case the shell was a straight one, not coiled 
round. Coiled forms occur in the Upper Silurian—the Goniatite, 
which is also found in the Coal Rocks. Specimens are on the 
table. In that genus the sutures are either simply lobed or shortly 
angulated, not sinuous. The Ceratite, you see, is an advance upon 
that form from an ornamental point of view. Of all the Ammonitide 
however, the Ammonites comprise by far the greater number of 
species, over 500 being already known. It is characteristically a 
fossil of the Mesozoic or Secondary Period, commencing with the 
Trias and ending with the Chalk. Each special rock group is 
marked by its special set of Ammonites, a bed of clay a few inches 
thick often containing species found neither above nor below it. 
These few inches, it should be remembered, represent the lapse of 
long ages, probably thousands of years, during which these particu- 
lar forms flourished. When the physical conditions changed the 
forms of life changed too, and developed into new species. Our 
own Gault clay is scarcely 100 feet thick, yet it has been divided 
into eleven beds, technically called ‘‘ horizons,” and nine of these 
are characterised by peculiar forms of Ammonite. 
It only remains for me now to barely allude to a few other 
_ chambered shells, more or less distantly related to the Ammonite, 
_ and all included in the same family. We find in East Wear Bay a 
long straight form curved at each end called Hamites, a smaller one 
g 
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; 
with the curved end in actual contact—Scaphites. In Ancyloceras 
we get one end spiral with separated whorls, and in Crioceras a 
wide open spiral. There are many others but time will not permit 
me to refer to them. I only trust that I have done something to- 
wards rendering your future rambles in the Bay more intellectually 
interesting than they may have been before. 
