TRIASSIC FISHES FROM SPITZBERGEN 
205 
corner between the dorsal and the anterior margins must have been rather acute-angled 
but it is not preserved in any of the specimens present. 
The suboperculum {Sop, text fig. 69; PL 26, figs. 1, 2; PL 27, fig. 1) is fairly small 
relatively to the operculum, probably scarcely more than a third of this. It is pentagonal 
in shape and its width is almost twice its height. One margin faces backward, one 
upward, one downward and two backward. When the bone is in situ the anterior margin, 
however, is turned so that it faces somewhat downward, the upper one so that it faces 
somewhat forward, the lower one so that it faces somewhat backward, and the upper 
of the two posterior ones so that it faces somewhat upwards. The anterior margin is 
weakly concave or almost straight and is separated from both the upper and the lower 
one by a right-angled corner. The lower margin is rather strongly concave and meets 
the lower of the two posterior ones in a somewhat rounded corner. The two posterior 
margins are fairly straight and meet each other in a backward pointing rather blunt 
corner. Finally the upper margin is sometimes almost straight, sometimes, on the other 
hand, it is concave at its anterior and convex at its posterior part. It meets the upper 
of the two posterior margins in an angular corner. 
The radii branchiostegi (. Rbr, text fig. 69; Pl. 26, fig. 1; Pl. 27, fig. x) were numerous, 
but their exact number cannot be established. They decrease gradually in length forwards. 
Farthest forwards at the angle between the two halves of the mandible there was 
presumably a small unpaired jugular plate (P. no). 
The sculpture on the bones of the visceral skeleton described agrees closely with 
that on the membrane bones of the cranial roof, i. e. it consists as a rule of somewhat 
lengthened and rather dense tubercles, mostly arranged somewhat parallel to the longi¬ 
tudinal axis of the animal. On the mandible the tubercles may become rather long, so 
that a longitudinal striation arises. PL 27, fig. 1, gives a detailed view of their arrangement 
on the operculum, suboperculum and radii branchiostegi. 
The branchial arches were ossified, but no details can be given about the different 
elements in them. 
Dentition. 
The dentition is known only on the maxillary and the dental. It consists as usual of a medial 
longitudinal row of larger and a lateral one of smaller teeth. All the teeth are conically pointed. 
The sensory canals of the head. 
The sensory canals of the head are comparatively fine and are, apart from possibly 
occurring «pit lines*, entirely enclosed in the membrane bones. They open outwards 
with a large number of fine pores, situated close together and directly laterally of the 
canals themselves, thus showing exactly the position of the latter. 
The most anterior part of the supraorbital canal, anteroir to the frontal, is not 
known. From the anterior end of either frontal the canal {ife, PL 26, fig. 2) has had its 
course backwards and somewhat medially, continuing to the anterior part of the parietal. 
The infraorbital canal is only partly preserved in its posterior part (, soc, Pl. 26, fig 2). 
Its position is the usual one, and it has, at least posteriorly, not communicated with 
the supraorbital canal. 
