LIFE IN THE PALEOZOIC EPOCH 463 



probable that the straight-chambered shell was first evolved, and 

 that this gradually underwent a process of coiling up, as a means 

 of facilitating locomotion. 



Marine Snails (Gastropoda) are abundantly represented among 

 the palaeozoic fossils, and it may be said generally that they be- 

 longed to the more primitive families, and were mostly of vege- 

 tarian habit. We also know that Land -Snails existed. The 

 Tusk-Shells (Scaphopodd] date back to this epoch, and Bivalves 

 (Lamellibranchia), the older ones of primitive type, were abundant. 

 Primitive Molluscs (Amphineura) were represented by Mail-Shells 

 (Chiton, &c.) of various kind. 



PALEOZOIC FISHES ( PISCES). There is no clear 

 proof of the existence of fishes in the earlier rocks 

 of the epoch, but later on they became abundant. 

 The teeth and spines of Sharks, &c. (Elasmo- 

 branchii], are common fossils, and the armour- 

 plated marine ancestors of Ganoids (Ganoidei) were 

 abundantly represented, as also forms in all proba- 

 bility ancestral to the Lung- Fishes (Dipnoi}. Many 

 palaeozoic fishes also belonged to orders that are 

 now extinct. Some of the oldest of the fish-like 

 forms, distinguished by a covering of shelly plates 

 on the front part of the body, also by the absence of lower jaws 

 and paired fins, were probably not really fishes at all, and have 

 been placed in a special group (Ostracodermata, fig. 1319) of 

 lower rank. 



PALAEOZOIC AMPHIBIANS (AMPHIBIA). These are represented 

 in the second half of the epoch by numerous species, all belonging 

 to an extinct order (Stegocephala), distinguished by the possession 

 of skins which were more or less armour-plated, especially on the 

 head. While some of these creatures were small, others attained 

 considerable dimensions. The footprints of some of them have 

 been preserved. The chief interest attaching to the order lies 

 in the fact that it was probably ancestral to Reptiles. It is also 

 worth while noting that some few members of the order (e.g. 

 Dolichosoma) were limbless and snake-like, suggesting a com- 

 parison with the recent Csecilians (Gymnophiona\ a widely dis- 

 tributed and in some respects primitive group, e.g. in the 

 possession of little bony plates in the skin. 



PALEOZOIC REPTILES (REPTILIA). A few fossil types from 



