THYNG: SQUAMOSAL BONE. 
417 
Summary. 
The development of the mammalian squamosal shows it to be a 
membrane bone which overlies the otic capsule and is at first inti¬ 
mately connected with the incus (quadrate) by a dense and fibrous 
stroma. Its dorsal margin is widely separated from the lateral border 
of the parietal, and only late in development comes into juxtaposition 
with the latter. Hence, it must be concluded that close association 
with the quadrate (incus) and otic capsule is the primitive relation 
of the squamosal, and, therefore, the most important criterion in as¬ 
certaining its homology in the non-mammalian vertebrates. Juxta¬ 
position with the parietal, on the other hand, is a relation which has 
been secondarily acquired, and one which has lead to erroneous 
views of homologies in the non-mammalian vertebrates. 
Moreover, we find in certain fossil vertebrates (Stegocephala) 
which in all probability directly or indirectly gave origin to the Mam¬ 
malia, not only the bone which corresponds perfectly with the squa¬ 
mosal as above defined, but a second bone also overlying the otic 
capsule between this squamosal and the parietal,— that is, in the 
interval which in the embryonic mammalian skull is unoccupied by 
bone. Hence, it seems reasonable to conclude that, in the phylo¬ 
genetic development of the Mammalia, this bone which may properly 
be called the supratemporal, has been lost, and its place has been 
bridged secondarily by the upward development of the squamosal. 
No bone has been found in the temporal region of the Caecilian 
skull homologous with the mammalian squamosal. The bone often 
so called, corresponds, in the authors opinion, more closely to a post¬ 
frontal. 
In the Urodelan skull, however, there is a bone which develops in 
membrane, and overlies the quadrate and otic capsule, secondarily 
reaching the parietal. This is the squamosal. 
As already stated there are two bones in the temporal region of the 
Stegocephalian skull external to the parietal. The more lateral, from 
its relation to the quadrate, the otic capsule, and the jugal, I have iden¬ 
tified as the squamosal. The more median is the supratemporal. 
Hence an inversion of the names of these two bones as ordinarily used, 
is necessary. This is also in accord with the priority of use of these 
terms. 
