392 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
Theromorpha or the Amphibian group Stegoeephala, the differences 
between certain members of the two being very slight. It is, of course, 
impossible to study the development of the bones in either of these 
groups and even the relations of the chondrocranium and visceral 
arches to the covering bones are hardly known in either. Hence any 
light which may be thrown upon the subject by the nearest living forms 
must have some value. The nearest relatives of the Stegoeephala 
are the Caecilians and the Urodeles, the Urodeles being in many 
respects admittedly degenerate and aberrant, while the Caecilians, 
as shown by many skeletal features, are decidedly the most Stegocepha- 
lian of all living Amphibia. Unfortunately the Caecilians, as will be 
detailed later, lack completely any element which can be homologized 
with the mammalian squamosal. 
The Squamosal in Urodeles. 
I have studied the development of the squamosal region in both 
Amblystoma and Amphiuma. In Amphiuma two bones are intimately 
related to the quadrate in development, one more external and more 
dorsal in position, the other more ventral and resting directly upon 
the quadrate. The latter will be considered later. The external 
and more dorsal bone (pi. 40, fig. 4), is membranous in origin. It 
overlies the upper external surface of the quadrate ( Qu .) and extends 
thence upward upon the lower part of the otic capsule, overlying the 
external (horizontal) semicircular canal. In its early stages it is sep¬ 
arated by a wide interval from the lateral margin of the parietal (Pa.). 
(C/. Kingsley, ’99, p. 165, fig. 144.) In other words, accepting the 
homology between incus and quadrate, this bone corresponds perfectly 
in position and relations to the mammalian squamosal with the single 
exception that, since no jugal is developed in the Urodeles, it lacks the 
jugal-squamosal articulation. 
This bone in the Urodelan skull has had numerous names proposed 
for it. It has been called the tympanique or tympanicum (Cuvier, 
Stannius, ’56, Wiedersheim, ’77, Gegenbaur, ’78, Hoffmann, etc.); 
a portion of the quadrate (os carre, Hallmann, Meckel); the temporo- 
mastoi'dien (Duges); the paraquadrate (Gaupp, Emerson, etc.) and 
the squamosal (Cope, Hasse, Huxley, ’74, Parker, Hay, ’90, Wieders¬ 
heim, ’93, Gegenbaur, ’98, Kingsbury, Gadow, and others). The 
term tympanic cannot be applied to it since it is already in use for the 
