THE TRIASSIC PERIOD. 221 
just named, it is still doubtful whether they are genuine 
reptiles, or perhaps Amphibia of the salamander kind. 
Their skeleton alone is known to us, and even this not 
perfectly. Now as we know nothing of the characteristic 
features of their soft parts, it is quite possible that the 
Proterosaurus and Rhopalodon were non-amnionate animals 
more closely allied to Amphibia than to Reptiles; possibly 
they belonged to the transition form between the two 
classes. But, on the other hand, as undoubted fossil remains 
of Amniota have been found as early as the Trias, it is 
probable that the main class of Amniota first developed in 
the Trias, that is, in the beginning of the Mesolithic epoch. 
As we have already seen, this very period is evidently one 
of the most important turning points in the organic history 
of the earth. The paleeolithic fern forests were then re- 
placed by the pine forests of the Trias period; important 
transformations then took place in many of the classes of 
Invertebrata. Articulated marine lilies (Colocrina) de- 
veloped out of the plated ones (Phatnocrina.) The Autechi- 
nidz, or sea-urchins with only twenty rows of plates, took 
the place of the palzeolithic Palechinidze, the sea-urchins 
with more than twenty rows of plates. The Cystidez, Blas- 
toidez, Trilobita, and other characteristic groups of Inverte- 
brata of the primary period became extinct. It is no 
wonder that transforming conditions of adaptation power- 
fully influenced the Vertebrate tribes also in the beginning 
of the Trias period, and caused the orig:n of Amniotic 
animals. 
If, however, the two Lizard and Salamander-like 
animals of the Permian system, the Proterosaurus and 
Rhopalodon, are considered genuine Reptiles, and conse- 
