472 BAUR. [Vou. Ill. 
orbitals, supratemporals, squamosals, jugals, quadratojugals, 
supraoccipital and paroccipital plates (epiotics). In some of 
the Stegocephalia another plate is found between the postorbital 
and the frontal (A/e/anerpeton) ; this we have to consider as the 
vestige of one of the more numerous plates of the ancestors of 
the Stegocephalia. 
In the oldest Testudinata we have also a complete bony cov- 
_ering, but the number of elements has been reduced from 22 in 
Melanerpeton to 12in Chelone. Thereare no distinct postorbitals, 
supratemporals, supraoccipital and paroccipital plates. This 
condition we still find in living forms like the Cheloniidz and 
Dermochelydida. From such forms the Chelydridz, Dermate- 
mydide, Staurotypidz, Cinosternide, Platysternide, Emydide, 
Testudinidz, Trionychia, developed by losing the connection 
between the squamosal and parietal,—in other words, by los- 
ing material from behind. But at the same time a reduction 
took place from below, between quadrate and jugal. The result 
was the formation of a more or less slender arch between the 
orbit and the quadrate. In a few cases this arch became 
entirely reduced, — Terrapene, Geemyda. 
In the Pleurodira two kinds of reductions took place; one 
from below, which destroyed the connection between orbit and 
quadrate, but retained that between squamosal and parietal. 
This we see in the Chelydide; in Chelodina even the connec- 
tion between squamosal and parietal has gone. The total 
absence of a temporal arch in Zerrapene and Gewmyda on one 
side, and of Chelodina on the other, is the product of two differ- 
ent kinds of reduction. The first is produced by reduction from 
below and behind ; the other by reduction from below only. 
In the Sternothzeridz and Podocnemididz a reduction takes 
place from behind and below, as in the Chelydrida. In none of 
the Testudinata is a true supra- or infratemporal fossa developed ; 
there is only one arch which represents the whole complex 
between parietal, frontal, and jugal, quadratojugal. 
The complete covering of the skull is for the first time inter- 
rupted in the Ichthyosauria and Aétosauria, by the appearance 
of a supratemporal fossa, which develops between parietal, 
squamosal, and the upper posterior border of the orbit. The 
bony arch below the supratemporal fossa which connects the 
orbit with the quadrate is now affected in two different ways: 
