124 PALAEONTOLOGY 



none at all. Only three living genera have an external 

 shell: that of Argonauta, the "paper nautilus," is of 

 a special type peculiar to it, and need not be considered 

 here ; that of Spirula is almost embedded in the mantle, 

 but is of the same general type as the completely external 

 shell of Nautilus (the " pearly nautilus "). This last is a 

 spirally-coiled univalve shell, divided internally into 

 chambers, and the same general type is found in an 

 enormous number of extinct Cephalopoda. As the 

 spiral coiling introduces a complication into the structure, 

 it will be convenient to describe first the similar but 

 straight shell of the extinct genus Orthoceras, filling up 

 the gaps in our knowledge by reference to the living 

 nautilus. There are a great many species of Orthoceras, 

 ranging in age from uppermost Cambrian (Tremadocian) 

 to Triassic, but as none of these is so common as to be 

 obtainable with certainty for examination, and it may 

 often be necessary to take several species to demonstrate 

 the full structure, the description here given is generic 

 instead of specific. 



i. Orthoceras. The shell (Fig. 36) is a cone ap- 

 proaching to a cylinder (sometimes circular, sometimes 

 elliptical in plan), and ending in a more or less blunt 

 rounded apex. In extremely rare cases there is found 

 attached to this apex what looks like a sraall shrivelled- 

 up bladder, and this is probably the horny equivalent of a 

 globular calcareous body in Spirula the protoconch, or 

 initial shell, secreted in early life. The main shell (or 

 conch as it is sometimes called) is divided internally into 

 chambers by partitions (septa), which are convex towards 



