339 
In the higher level of the sacculus, the elongation 
of the external semicircular canal, and in the division 
of the auditory nerve Polypterus shows distinct Uro- 
delan characters. 
2) Primordial cranium. The posterior external angle of the 
skull is occupied by a bone which has been called “opisthotic”. It 
serves for the attachment of many muscles and supports the hyo- 
mandibular and 1“ pharyngobranchial besides protecting the hinder 
portion of the external and posterior semicircular canals. It shows 
an important process posteriorly which becomes large in old beasts. 
This process is probably the “craniospinal” process of Sturgeons (PAr- 
KER) but it corresponds to a similar process in Siren and other Uro- 
deles. In brief the bone is not an opisthotic or intercalare of fish 
but corresponds to the “Petrosum” of Urodeles. 
In front of the vertebra which has been drawn into the skull lies 
in young specimens a small separate piece of cartilage surrounded by 
a thin shell of bone. The chorda dorsalis passes up behind it curving as 
in the developing Lepidosteus and Sturgeon and then terminates above 
it. This piece corresponds to the basioccipital cartilage described by 
WIEDERSHEIM in Menobranchus. 
As stated by PARKER the supraorbital cartilage corresponds to 
a process in Siren and the similarity of the “sphenoid”, which is very 
variable, to the orbitosphenoid of Urodeles has already been noticed 
by Traquair and BRIDGE. 
There is no mesethmoid ossification. The cartilage in front of 
the nasal septum projects as a horizontal ledge above and below run- 
ning out however more solidly and further below. Between the two 
projections runs across the large mucous canal enclosed in the dermal 
ethmoid. The inferior projection is a mere rudiment of the enormous 
rostrum of Lepidosteus and the Sturgeon, and a similar inferior pro- 
jection is very well seen in Siren. 
Around the olfactory openings at the upper and also at the 
posterior edges little projections of cartilage are seen, almost detached. 
Their meaning will be refered to later. In their passage from the 
orbit the ophthalmicus superficialis and profundus pass through a 
large canal in the ectethmoid and then through a foramen at the 
posterior, superior angle of the nasal capsule into the latter. This 
foramen is present in Urodeles. 
The supracranial fontanelle is partially roofed over at its centre 
by a thin independent sheet of cartilage. In Amia the roofing is 
complete. In Anura there is a partial roofng. The affinity of 
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