THYNG: SQUAMOSAL BONE. 
399 
posteriorly and conjectured that this was a quadratojugal which had 
fused with the quadrate. Gaupp (’94, p. 102, footnote) writes: “Bei 
Triton wo ein Quadrato-jugale fehlt, beginnt trotzdem die Verknoch- 
erung des Quadratum an der vorderen ausseren Ecke als perichondrale 
Knochenauflagerung — ein Verhalten das geeignet scheint, dem 
Quadrato-jugale noeh cine besondere Bedeutung fiir die Verknoch- 
erung des Quadratum zu zukennen.” Kingsbury in 1903 and again 
in 1905 called attention to what seemed to be a separate ossification 
in Necturus, arising on the outer surface of the cartilaginous quadrate, 
and at first only partially covered by the squamosal. This ossifica¬ 
tion, which he first called the subsquamosal process of the quadrate, 
but later the “bone X,” was at first distinct from the quadrate cartilage. 
Soon, however, it fused with 
the cartilage and from this 
place a zone of ossification 
extended around it, forming 
a ring of bone. In the larval 
Spelerpes practically the same 
conditions occurred except 
that in the adult this element 
was no longer distinguishable 
from the quadrate, all parts 
of which, with the exception 
of the articular surfaces, had 
ossified. From observations 
on Amphiuma, Amblystoma, 
and Desmognathus he con¬ 
cluded that this mode of ossi¬ 
fication was at least frequent 
among the Urodeles. 
My observations on Nec¬ 
turus, Amblystoma, Triton, 
and Amphiuma confirm 
Kingsbury’s account of the 
existence of this separate ossi¬ 
fication. Figure D shows 
this bone (Qj.) as it occurs 
in Amphiuma. Now, since 
this bone is intimately asso¬ 
ciated with the quadrate, descending on its external surface nearly 
s\o A 
Fig. D. — Transverse section through the otic 
region of Amphiuma, showing the quadratojugal 
ossification arising upon the external surface 
of the quadrate and covered to a large extent 
by the squamosal. OC., otic capsule; Qj., 
quadratojugal; Qu., quadrate; Sq., squamosal. 
