244 B. NATURAL HISTORY. 
It will be observed, by this table, that the most specialized Rep- 
tilian order, the Ophidia, appeared last in time in the Eocene period ; 
and that those which constitute the line of connection with the gen- 
eralized reptiles appeared earlier as they approached the latter, — 
the Pythonomorpha in Cretaceous, and Lacertilia in Jurassic times. 
The Reptilian groups most specialized in bird characters (Ornitho- 
-sauria and Dinosauria) appear on the other hand very early ; the first 
and most Mammalian also,—the later of the two,—in Jurassic 
Beds. The Trias gives us in the Anomodontia and Ichthyoptery- 
gia, the two most generalized and lowest orders; while their 
contemporary, the Rhynchocephalia, almost as much generalized in 
Reptilian features proper, was already represented in the Trias. 
Strangely enough this order yet exists in the living Sphenodon of 
New Zealand. The Crocodilia, rather specialized in bird charac- 
ters, accompanies the last in this wonderful persiatemey, beginning 
also in the Trias. 
The inquiry as to the truth of the proposition that the more - 
ancient types of animals are more generalized, and therefore more 
embryonic in the characters of a special nature* which character- 
ized groups later introduced, is answered in a very imperfect way 
in the affirmative. It is like the shadow of a truth whose substance 
will shortly come before us. But when we come to compare the 
subdivisions of the orders themselves with each other, and with 
those of other orders, as we pass backwards in time, the weight of 
the affirmative answer to the above proposition is greatly increased. 
The oldest Ophidia are bozform, therefore approaching Lacertilia 
and Pythonomorpha. The oldest Tortoises have generally the most 
incomplete carapace and plastron; among them the Psephoderma ° 
allied to Sphargis, without carapace, and thus the most lizard-like of 
the order. The Lacertilia of European Jurassic strata are, some of 
them at least, acrodont, apparently Pachyglossa (e. g., Acrosaurus), 
and, as such, nearer the Rhynchocephalia, which preceded them in 
time. The position of Homorosaurus and Piocormus is not deter- 
minable, as the dentition cannot be understood from the descriptions 
and figures of Wagner. The form of the mesosternum of the former 
refers it to either the Pachyglossa or Iguania, as I understand those 
groups. It may be assumed that since the order Lacertilia has 
‘diverged from the: line of other Reptilia, while it took on in its 
special peculiarities it lost in the features characterizing the main 
* The identity of these two propositions has not always been noticed by 
authors. 
