23 



this on the right hand figure 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 Nautilris. 

 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 Eocks. 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 Ammnnitidce 

 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 distanily 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 

 with the curved end in actual contdiGi—Sca'pliites. 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. 



