118 EXTINCT MONSTERS 
forms as Eryops (Fig. 20), Empedias, and Dimetrodon. As these 
are not completely known, we have refrained from attempting 
to restore them. Perhaps the most remarkable is the Dimetrodon ! 
(see Fig. 33) belonging to the order Pelycosauria. Not only 
was the skull provided with formidable tusks, but the vertebrae 
of the back present a new feature in having very long neural 
spines. In one species the height of the spine is actually more 
than twenty times the length of the centrum! According to 
Professor Cope, these spines formed a kind of elevated fin on the 
back, of which it is difficult to imagine the use; but then there 
are many living animals with bony structures which, if only 
known in a fossil state, would greatly puzzle every one. 
Plate XIII. shows a photograph of a restoration of the allied 
Naosaurus from Permian strata of Texas, for which the writer 
is indebted to Professor H. F. Osborn. We will take Protero- 
saurus next. 
The skull is more lizard-like than crocodilian. There are 
mainly two interpretations of this fossil. (1) Cuvier and Huxley 
class it unreservedly with the lizards. (2) Von Meyer and Owen 
consider that it represents a new type of reptile. Von Meyer 
published an elaborate monograph on the subject, with nine folio 
plates (1856). Professor Seeley also considers that it shows no 
particular affinity with any living group of reptiles. It evidently 
came of a very ancient stock. With regard to its habits, Owen 
concluded that it was an aquatic animal. He says, “ The strength 
of its neck and head, and the sharpness of its teeth, enabled it to 
seize and overcome the struggles of the active fishes of the waters 
which deposited the old Thuringian copper-slates.” There can be 
but little doubt, however, that it was capable of walking on land 
as well as of disporting itself in the water Certain ganoid fishes 
1 Greek—dis, double ; metron, measure ; odous, tooth. 
