TRIASSIC FISHES FROM SPITZBERGEN 
223 
mutually independent, small tooth-bearing plates. Hyomandibular situated obliquely 
with a large processus opercularis. Mouth large. Preoperculum wide of a rather trian¬ 
gular shape; operculum large, high. Suboperculum small, low. Branchiostegal rays few 
or at least not very numerous. Median jugular plate very large. All the external mem¬ 
brane bones of the visceral skeleton with a sculpture of fine, fairly short, longitudinally 
arranged striae. Dentition probably somewhat robust with both pointedly conical and 
bluntly concial teeth. The sculpture of the shoulder girdle like that of the opercular 
bones. Dorsal fin probably opposite the ventral fins. Anal fin smal, situated far back¬ 
ward. Caudal fin deeply cleft. Pectoral and ventral fins weakly developed, the latter 
probably situated about half way between the pectoral fins and the anal fin. Lepidotrichia 
of all fins, apart from the two or three most anterior ones in the pectoral fins, closely 
jointed. Fulcra present, at least on the pectoral fins, but small. Scales higher than 
wide only on the very anterior part of the lateral surfaces of the abdominal region, 
decreasing in height in a caudal, dorsal and ventral direction, so that the width becomes 
predominant. Towards the dorsum a number of scales in each vertical row are very 
low, and even ventrally there are low scales. The scales often rather thin. At least 
the scales of the abdominal region have their covered area wide and the antero-dorsal 
corner somewhat produced upwards. On the dorsal margin an articulatory spine is 
developed. The sculpture on all the scales consists partly of longer, partly of shorter 
striae. The longer ones cross the whole exposed surface of the scales from the anterior 
to the posterior margin, in most cases sloping somewhat downwards and backwards. 
The shorter ones are always fine, numerous, and developed exclusively in the immediate 
neighbourhood of the posterior margin between the posterior ends of the longer ones. 
It is easy to see that the genus Acrorhabdus has most of its characters in common 
with Boreosomus, to which it is undoubtedly closely related. It is, howewer, clearly 
distinguished from Boreosomus, especially by the sculpture of the head and the scales, 
and by the fact that almost all the lepidotrichia in the pectoral fins are jointed through¬ 
out their length. We shall see that differences between the two genera can also be 
observed with regard to the development of a number of bones in the head. 
Like Boreosomus, Acrorhabdus shows, at least with regard to the shape and sculpture 
of the scales, certain agreements with Acrolepis. 
It is beyond all doubt that both Boreosomus and Acrorhabdus represent among the 
Palaeoniscids a special group, characterized by the position of the dorsal fin and certain 
specializations in the visceral skeleton. As I have already been able to show to some 
extent (pp. 2i3—214), this specialization has, at least partly, proceeded in the same 
direction as in higher Ganoids and Teleosts. According to what I am able to show 
below, the gap between Boreosomus and Acrorhabdus on the one hand the Catopterids 
on the other must really have been rather slight. 
Acrorhabdus bertili n. sp. 
(PI. 3i; PI. 32, figs. 2—5.) 
The determinable material of this species at my disposal consists of only two incomplete 
specimens (P. 172, P . 174). Besides these I include here too, though with hesitation, a 
couple of fragments with scales (P. n 3 , P. 175) and the remains of a caudal fin (P. i 3 s )l 
