THYNG: SQUAMOSAL BONE. 
389 
temporal and the substance of his account may be repeated here. 
The squamosal is developed in membrane upon the outer side of the 
periotic cartilage. It is, at first, a mere rod, the zygoma, with an ex¬ 
panded portion, the squama temporalis. Its lower edge is at first 
nearly straight but it soon develops a curved process (the margo 
tympanieus or posterior process of the adult human temporal) down¬ 
wards behind the auditory meatus and between the tympanic ring 
and the periotic bones. 
Development of the Squamosal in Mammals. 
My studies of the development of the squamosal in the mammals 
are based upon serial sections and wax reconstructions of embryo 
pigs. In most respects my results are but confirmations of the ac¬ 
counts of others, but they show certain points which have not been 
sufficiently valued or which have been overlooked by others, and 
which, it seems to me, have great importance in determining the 
homologies of this bone in other groups. 
The squamosal (pi. 39, fig. 1, Sq .) arises as a membrane bone, con¬ 
sisting of a zygomatic process in front and a small squamous portion 
behind. The zygomatic process articulates anteriorly with a simul¬ 
taneously developed jugal or malar (J.) while the upper part of the 
squamous portion overlies the lateral border of the cartilaginous otic 
capsule directly in front of the ampullar ends of the anterior and the 
horizontal (external) semicircular canals. Its lower half extends 
ventrally as a slender splint (the margo tympanieus of Henle) down 
below the capsule and over the processus brevis of the incus. This 
portion deserves especial attention since there seems to be an intimate 
relation between this margo tympanieus and the incudal element. 
As shown in figures 2 (pi. 39) and 3 (pi. 40) representing sections of 
a pig of 72 mm. total length, a dense and fibrous stroma unites the 
squamosal with the incus. This condition, which has been overlooked 
by previous writers, I regard as very important as indicating a close 
primitive relation between incus and squamosal to which reference 
will be made again later. 
Another important morphological character which deserves special 
emphasis is that, in its early development, the squamosal is widely 
separated from the parietal, for there is a wide gap between the dorsal 
