OSTEOLOGY. 293 
Great anteater (Myrmecophaga), the nasals have kept pace with the growth 
of the snout and appear at the dorsal side of the nasal aperture. Apparently 
in Ornithorhynchus these bones are not thus excluded by the premaxillaries 
which in this animal form the sides and anterior boundary of the opening. 
The series of skulls of the Proechidna shows certain features of interest 
in regard to the order in which the sutures close. In the youngest specimen 
(M. C. Z. 7,009) all the chief sutures are discernible with little difficulty, and 
the broad jugal bones are so slightly fused with the cranium that they came 
entirely away in cleaning. The sutures of the occipital portion of the skull 
are especially clear, and the small median interfrontal is distinctly outlined. 
In a second specimen (M. C. Z. 7,010) of practically the same size, the premaxil- 
laries are thoroughly fused in the dorsal midline, from the tips of the nasals 
to the nasal aperture, and the nasals are fused along their median line of contact 
though still separate laterally from the surrounding bones. The interfrontal 
is also with great difficulty to be traced, and the anterior edge of the large inter- 
parietal is beginning to coalesce medially with the frontal. At the occipital 
region of the skull the condylar portion of the basioccipital has become fused 
with that of the exoccipital though elsewhere the occipital sutures are broadly 
open. The parietal sutures are disappearing and the jugal is merging into the 
cranial wall. The other sutures are still traceable. A third skull (M. C. Z. 
12,415) has attained adult size and shows an advance in ossification. In dorsal 
aspect the only sutures visible are:— the lateral and dorsal sutures marking 
off the exoccipitals, the lateral suture bounding the supraoccipital, the suture 
between the jugal and the maxillary process of the zygomata, and the sutures 
delimiting the maxillaries from the frontals, nasals, and premaxillaries. The 
nasals are solidly fused together and to the premaxillaries, though these are 
still distinct laterally and ventrally from the maxillaries. Conspicuous rough- 
ened furrows are developed on the vertex of the cranium for the attachment 
of muscles, and the posterior expansions of the jugals are solidly fused with the 
skull. In ventral aspect, the posterolateral boundaries of the basioccipital 
are quite obliterated but elsewhere the sutures delimiting this bone are still 
open, so that in appearance the basi- and exoccipitals are fused into a single 
bone whose boundaries are everywhere distinct. The bones of the palatal area 
are still unfused, but those of the orbit have become well soldered together. 
As in the Echidna the palatal branch of each premaxilla becomes much 
drawn out posteriorly and extends as a narrowly tapering process to the origin 
of the zygomatic portion of the maxilla. This bone seems to increase slightly 
