THE GEOLOGICAL MIDDLE AGE. 165 
Perch, Herring, Smelts, etc., compare with our 
larger Fishes ; hut, whatever their size or form, 
all the Fishes of those days had the same hard 
scales fitting to each other by hooks, instead of 
the thin membranous scales overlapping each 
other at the edge, like the common Fishes of 
more modern times. The smaller Fishes, no 
doubt, afforded food to the larger ones, and to 
the aquatic Reptiles. Indeed, in parts of the in- 
testines of the Ichthyosauri, and in their petrified 
excrements, have been found the scales and teeth 
of these smaller Fishes perfectly preserved. It is 
amazing that we can learn so much of the habits 
of life of these past creatures, and know even 
what was the food of animals existing countless 
ages before man was created. 
There are traces of Mammalia in the Jurassic 
deposits, but they were of those inferior kinds 
known now as Marsupials, and no complete speci- 
mens have yet been found. 
The Articulates were largely represented in 
this epoch. There were already in the vegetation 
a number of Gymnosperms, affording more favor- 
able nourishment for Insects than the forests of 
earlier times ; and we accordingly find that class 
in larger numbers than ever before, though still 
meagre in comparison with its present represen- 
tation. Crustacea wero numerous, — those of 
the Shrimp and Lobster kinds prevailing, though 
