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ERIK A : SON STENSIO 
that with regard to the development of the orbitotemporal region Polypterus represents 
a far less specialized type than B. mougeoti, which has evidently started to develop in 
the direction of Lepidosteus, Amia and Teleosts. For in Polypterus the cranial cavity not 
only extends trough the whole orbitemporal region but has also about the same width 
dorsally as ventrally. The floor of the cranial cavity in Polypterus is therefore compa¬ 
ratively wide and the cranium gets a certain platybasic character. Under these circum¬ 
stances the interorbital wall is of course relatively thick. It is also noteworthy that 
both the myodome and the fenestra optica are absent. The distance between the foramen 
opticum and v. pituitaria is long and not shortened as in the Teleostei and in Amia, 
Lepidosteus and Birgeria. In this respect Polypterus thus resembles Dictyonosteus and the 
Elasmobranchs. x ) 
It is also evident that with regard to the general characters of the orbitotemporal 
region B. mougeoti is considerably more specialized in the direction of the Teleostei than 
the sturgeons. 
Among fossil ganoids only Dapedius and Lepidotus are so far known to any 
extent with regard to the development of the orbitotemporal region. As far as one can 
understand, the general characters of this region have in them been fairly like those 
in the Teleostei. Ossifications have been present but no details are known as to the 
boundaries of these (Woodward, i 8 g 3 b, pp. 560—-565; 1915, p. 3 g; Frost, igi 3 , pp. 219—222). 
The fenestra optica is well developed. 
Among the Crossopterygians Dictyonosteus (Stensio, 1918 c) shows a striking resem¬ 
blance to B. mougeoti by its large sphenoid, which extends into the most anterior part 
of the labyrinth region, and this is also the case to a certain extent which the Coela- 
canthids, where the basisphenoid corpus is situated in the labyrinth region in a similar 
way to the pars basisphenoidea of the sphenoid in Dictyonosteus. For further details 
with regard to this I refer to my description of the Coelacanthids in this work above 
(pp. 56—60, 91—92, 1 32 ). It is, however, evident that with regard to the development 
of the orbitotemporal region both Dictyonosteus and the Coelacanthids show in other 
respects rather different conditions from B. mougeoti, and that in this respects they pro¬ 
bably resembled Polypterus in essentials. 
We thus see that with regard to the development of the orbitotemporal region 
Birgeria mougeoti has started to develop in the direction of the Teleostei, but that it also 
has certain characters in common with Polypterus and Crossopterygians. It also seems 
to me fairly evident that these characters are primitive and indicate a close relationship 
between the primitive Actinopterygii and the Crossopterygii (cf. pp. 137—140 above). 
It is also worth mentioning in this connection that our view of the relationship 
of the Polypterids to other Teleostomes has become rather clearer because of the more 
exact knowledge gained with regard to the development of the orbitotemporal region 
in B. mougeoti on the one hand and in Dictyonosteus and Coelacanthids on the other. It 
certainly seem evident that with regard to the orbitotemporal region the Polypterids 
resemble Dictyonosteus and the Coelacanthids very much, but there seems to be no 
reason to refer them to the Crossopterygii because of this, as, on account of the 
*) In Calamoichthys there are in the orbitotemporal region according to Supino (19x4, p. i83) a paired basi¬ 
sphenoid, alisphenoid and orbitosphenoid, but no details are given as to their extension or relation. 
