No. 3.] VERTEBRATE-SKULL. 473 
1. The broad single arch remains single, but becomes more 
and more slender and can be interrupted. Plesiosauria, 
Theromora, Mammalia, Squamata (Lacertilia, Pythonomorpha, 
Ophidia). 
2. In the broad single arch appears another opening, the 
infratemporal fossa forming an upper and lower arch which 
connects the orbit with the quadrate: Rhynchocephalia, the 
whole archosaurian branch (Crocodilia, Dinosauria, Pterosauria), . 
Birds. 
Until now the Squamata had been considered by a large 
number of anatomists as derived from the Rhynchocephalia, 
and I have held the same view. The Squamata were looked at 
as Reptiles, in which the lower temporal arch of the Rhyncho- 
cephalia had disappeared. But this is not the case. It seems 
very much more probable that the Squamata never possessed an 
infratemporal fossa, but that the broad arch was reduced from 
below in the same way as in the Testudinata ; and that the old 
opinion of Hallmann, Hollard, Owen, that the squamosal of the 
Squamata is the homologue of the quadratojugal, and the supra- 
temporal or mastoid of the Squamata the homologue of the 
squamosal of the Archosaurian branch, is the correct one. In 
all Reptilia, if no reduction has taken place, there are two 
elements between quadrate and parietal: the only exception is 
found in the Ichthyosauria. In these a third bone is inserted, 
which has to be considered as an original part of the upper 
bone. The same element is found in the Stegocephalia. In all 
these forms in which a quadratojugal has been recognized, there 
is never a “supratemporal” (as seen in the Lacertilia) ; in all 
forms in which this supratemporal was found, there is no quad- 
ratojugal. From a careful comparison of the skulls of the 
different groups of Reptilia, I reach the conclusion that the 
supratemporal or mastoid (opisthotic, Cope) of the Squamata 
is in fact the squamosal, and the bone called squamosal the 
quadratojugal. 
In the Iguanida, Agamidze, Lacertide, Anguidz, Varanide, 
both bones are well developed; in the Helodermatidz the 
squamosal (supratemporal, Parker; opisthotic, Cope) is large, 
the quadratojugal (squamosal, Parker) very small; in the Ophidia 
the quadratojugal has disappeared entirely. In the Tejidz the 
squamosal becomes reduced and united in some forms with the 
