150 
THE GEOLOGICAL MIDDLE AGE. 
margins, before the type disappears, as to make 
an intricate tracery of very various patterns on 
the surface of these shells. The most conspicu- 
ous type of Articulates continues as before to bo 
that of Crustacea; but Trilobites have finished 
their career, and the Lobster-like Crustacea make 
their appearance for the first time. It does not 
seem that the class of Insects has greatly in- 
creased since the Carboniferous epoch ; and 
Worms are still as difficult to trace as ever, be- 
ing chiefly known by the cases in which they 
sheltered themselves. Among Vertebrates, the 
Fishes still resemble those of the Carboniferous 
epoch, belonging principally to the Selachians 
and Ganoids. They have, however, approached 
somewhat toward a modern pattern, the lobes of 
the tail being more evenly cut, and their general 
outline more like that of common fishes. The 
gigantic marsh Reptiles have become far more 
numerous and various. They continue through 
several epochs, but may be said to reach their 
culminating point in the Jurassic and Cretaceous 
deposits. 
I cannot pass over the Triassic epoch without 
some allusion to the so-called bird-tracks, so gen- 
erally believed to mark the introduction of Birds 
at this time. It is true that in the deposits of 
the Trias there have been found many traces of 
footsteps, indicating a vast number of animals 
