338 



GEOLOGICAL BIOLOGY. 



time. At least, of the structures preserved to tell us the 

 story these two are the extremes — one, Orthoceras, a simple 

 slender cone, straight, and with regular septa dividing it into 

 chambers, and with a central siphuncle ; the other. Nautilus, 

 a closely coiled disciform shell, with siphuncle also central, 



Fig. iio. — Theoretic sections throuKh the middle of the shells to show the variations in the curva- 

 ture and coiling of Paleozoic Cephalopod shells. A, Clymenia ; />', Nautilus; C, Nautilo- 

 ceras; Z), E^ Aploceras ; F^ Orthoceras; (7, Melia ; //, /, Cyrtoceras ; y, Gyroceras ; K^ 

 Ophidioceras ; Z., Cryptoceras ; J/, (ioniatites. (After Gaudry.) 



outer chamber large, and whorls with ventral side out. The 

 two features which best express in these shells the amount or 

 degree of differentiation are, the amount and direction of 

 the curvatures of the shells and the position of the siphuncles. 



