TRIASSIC FISHES FROM SPITZBERGEN 167 
pp. 75—79; Sagemehl 1884, pp. 215—217; 1885, pp. 85—87; 1891, pp. 574—575; Gaupp 1905, 
pp. 667, 669; Allis 1909 a, pp. i 83 —208; 1914 a, b; 1918 a). 
Having examined the pars basalis of the sphenoid rather completely let us turn to its 
partes laterales. These issue in a dorso-lateral direction from the pars basalis. Their ventral 
part is fairly thin, but their thickness increases dorsally and soon becomes rather con¬ 
siderable, still farther dorsally suddenly beginning to decrease again (text fig. 64 A, B). 
Towards their posterior ends the thickest part is situated half way between the base of the 
neurocranium and the dorsal surface, towards their anterior ends this part is displaced gradu¬ 
ally upwards, and finally at their most anterior parts it is situated quite close the to cranial roof. 
As has already been pointed out, the partes laterales of the sphenoid extend considerably 
farther forward than the pars basalis, while, on the other hand, they do not reach quite 
so far backwards as this. The anterior margin of either of them is almost vertical. The pos¬ 
terior one, on the other hand, passes off obliquely backwards and downwards in its upper 
part, becoming vertical in its lower part. By a marked and rather deep and wide sinus 
(r, text fig. 63 A,B, C; PI. 20, fig. 6; PI. 21, figs. 1— 3 ; PI. 22, figs. 1, 3 ) the posterior margin 
is divided into a dorsal (b) and a ventral (c) process, both of which are blunt and 
robust. The ventral process (c), as already mentioned above, is well marked off ventrally 
from the basal parachordal plate (pc) by the deep incisur fi , penetrating from behind. 
In the pars lateralis of each side we can distinguish four surfaces, well marked 
off from each other, a medial, a dorsal, a latero-dorsal and a lateral one, of which the 
last in fact really faces ventro-laterally. The medial and the lateral ones are the largest, 
the dorso-lateral one the smallest. 
The dorso-lateral surface (text fig. 63 A; PI. 21, figs. 2, 3 ) is fairly wide at its caudal 
end and meets the medial surface here in a distinct edge. Anteriorly it grows continously 
narrower. It is also separated from both the dorsal and the lateral surfaces by distinct 
longitudinal edges, the most prominent of which is the one between it and the lateral 
surface, which has two rounded corners (a, a.j) in its posterior half. This edge corresponds 
to the place where the partes laterales again begin to decrease in thickness upwards, 
and thus at each point of its extension it coincides with the position of their thickest 
place, as is shown by text figs. 64 A, B. 
The dorsal surface is plane; the lateral one is concave both in the direction from 
front to back and from above to below, the medial one only in the latter direction. The 
latero-dorsal surface, apart from smaller and very shallow concavities, is convex from 
the anterior to the posterior end. 
The dorsal surface was directly covered in its posterior half by the membrane bones 
of the cranial roof. Its anterior half, on the other hand, was separated from these by a 
very thin layer of cartilage. The latero-dorsal surface has been connected by cartilage 
throughout its extension and cartilage has also been attached to the medial one at its 
anterior and posterior ends and along a longitudinal zone close to the dorsal margin 
(text fig. 64). Further we find that cartilage must have been attached to the partes 
laterales along the posterior and anterior borders and along the ventral border, in the 
part situated in front of the pars basalis. J ) 
*) The surfaces to which cartilage has been attached are very easy to identify by their appearance and there 
is consequently no doubt as to the relations of the sphenoid to surrounding cartilaginous parts. 
