FOSSIL ANIMALS 



343 



Evolution of the Cephalopods. — ^An excellent fossil record among the 

 invertebrates has been established for the tetrabranchiate (four-gilled) 

 cephalopods (Mollusca), already used to illustrate the biogenetic law 

 (page 255). This branch of the cephalopods is represented today by 

 Nautilus, which lives in a coiled shell, externally resembling a snail shell. 

 The animal lives in only a small portion of the shell near the aperture. 

 The rest of the shell is divided by partitions into a number of chambers, 

 from which the animal is excluded except for a small stalk that extends 

 back through all of them. These partitions, or septa, represent the 

 positions occupied by the animal earlier in its life. As the body grows. 



Fig. 293. — Diagrams of sutures of cephalopods, slightly more than half shown, 

 orthocone; B, nautiloid; C, goniatite; D, ceratite; E, ammonite. 



A, 



it moves periodically forward into the wider part of the shell and secretes 

 a partition behind itself each time it moves. 



Tetrabranchiate cephalopods have been found as fossils in Cambrian 

 rocks. They became fairly abundant in early Ordovician time. At 

 that time, unlike the modern Nautilus, their shells were straight cones 

 (orthocones) . All later forms appear to have descended from these 

 orthocones. 



The course of evolution was as follows. The shell soon began to 

 bend and in many forms became closely coiled in flat spiral form (Fig. 

 210) like the shell of some snails. Owing to their resemblance to Nautilus 

 these animals are called nautiloids. They were very abundant in Silurian 

 time. Up to this period the septa across the shell were flat and saucer- 

 like, and the sutures, the lines of junction of the septa with the wall of the 

 shell, were nearly straight or only slightly curved. Later the septa 



