56 



FOSSILS OF LIASSIC SYSTEM. 



Fig. 79. A'mcula in- 

 tequiva'lvis. 



Fig. 78. Ammonites Walcoti. 



38. We also find in this group various species of Trigo'nia, 

 (fig. 80), which appear to have existed in all parts of the depo- 

 sit ; but the species, which perhaps furnish very important charac- 

 teristics, have not yet been studied sufficiently in relation to the 

 grouping. They extend through the o'olitic series to the chalk 

 fo mation. 



Fig. 80. Trigo'nia clavella'ta. 



39. We find too, in the lias for the first time, in ascending 

 through the crust of the earth, those singular saurians whose ske- 

 leton at the same time reminds us of lizards, crocodiles, fishes and 

 mammals ; their feet, which are in form of paddles, show they 

 were aquatic in their habits : such are the Ich'thyosau'rus, (,fig* 

 81), some of which were twenty-five feet in length; the Plei'sio- 

 sau'rusj (Jig. 82), some species of which are nearly fifteen feet 

 long. 



18. Are any species of Trigo'nia characteristic of any part of the Lias ? 

 3D. What is an Ich'thyosau'rus ? What is the lowest stratum in which it 

 is found ? What is the Plei'siosau'rus ? 



