THE FAIRY SAILOR AND HIS COUSINS. 119 



turn, the position of the siphon, and the plan of enrollment. 

 The septum may be plain, or angulated, or lobed, or folia 

 ted around its outer margin. The siphon may be external, 

 or internal, or central. The enrollment may be close, loose, 

 half-coiled, arcuate, or straight. Of how many combina 

 tions, three in a set, do these characters admit ! And yet 

 almost every possible combination has been realized in the 

 history of the world. In the earliest periods were the spe 

 cies with simple septa and straight 

 shells (orthoceratites,Fig. 47) ; next 

 came those with simple septa and 

 coiled shells (Nautili, Fig. 48); 

 then those with angulated septa 

 and coiled shells ( Clymenia, Fig. 

 49) ; then those with lobed septa 

 and coiled shells ( Goniatites, Fig. 

 50) ; lastly appeared those with fo 

 liated or very complicated septa, 

 Fig. 52. Ammonites caiiaiicuia- w ith their straight (Bciculites, Fig. 



tus. A chambered shell of 



the Mesozoic Ages. 51) ? arcuate (Hamites), closely 



(Ammonites, Fig. 52), and variously coiled forms. So we 

 see that in the various ages of the world, some type of 

 &quot; chambered shells&quot; has constituted a leading character- 



Pig. 53. Plans of Septa among different families of Chambered Shells. 



a. Septum in Nautilus family, b. Septum in Clymenia family, c. Septum in 



Goniatite family (Goniatites Marshallensis). d. Septum in Ammonite family. 



