270 
ERIK A: SON STENSIO 
the orbital entrance. The mandibles lack coronoid processes. The hyomandibular is more 
or less oblique and has a processus opercularis. The preoperculum is a large triangular 
plate, extending far in front over the cheek. The operculum and the suboperculum 
both well developed, the latter often high. Interoperculum lacking. A median jugular 
plate is present. Dention often adapted for crushing. The supraorbital canal has its 
posterior end in the parietal; posteriorly it does not anastomose with the infraorbital canal; 
anteriorly, on the other hand, there is an anastomosis through the antorbital, laterally 
of the nasal opening, between the two. The infraorbital canals have an ethmoidal commissure 
anteriorly through the rostrals. The preopercular canal runs along the posterior margin 
of the preoperculum. The cranial section of the lateral canal has a supratemporal 
commissure. On the parietal, the anterior, middle and posterior head-lines of pit organs 
are to be found at least in certain forms. The chorda dorsalis persistent and the ossi¬ 
fications of the neural and haemal arches probably rather weak. The caudal fin abbreviate- 
heterocercal or almost homocercal, always with its lepidotrichia jointed throughout. In 
both the anal and dorsal fin the lepidotrichia are present in nearly or quite the same 
number as the endoskeletal radials; in these fins each endoskeletal radials with only 
one long ossification; the lepidotrichia sometimes quite jointed, sometimes more or .less 
unjointed proximally. The shoulder girdle has a large cleithral and a small clavicle 
(infraclavicula). The lepidotrichia of the pectoral fins are usually unjointed for a long 
distance proximally. The ventral fins are short-based; their lepiciotrichs are jointed 
troughout or else unjointed for a bit in their proximal part. Fulcra are often rather 
powerfully developed on all the fins. Scales, thick and robust with a covering of ganoine. 
In some forms an accessorial lateral line dorsally of the main one is found, extending 
from the neck towards the basis of the dorsal fin. 
It is certain that, especially with regard to the development of the head and the 
shoulder girdle, the Catopterids are closely related to the Palaeoniscids, while, on the other 
hand, with regard to the development of the fins, they have attained a higher specialization 
than these. Although, as we have seen, the degree of specialization differs in this respect in 
different forms, still they all have, unlike the Palaeoniscids, an abbreviate-heterocercal 
caudal fin and about the same or practically the same number of lepidotrichia as endo¬ 
skeletal radials in both the dorsal and anal fins. To this may be added that the ventral 
fins are, presumably as a rule, considerably more short-based than in the Palaeoniscids. 
When these characters thus occur together it becomes necessary to separate the 
Palaeoniscids and Catopterids from a systematic point of view, even although it seems 
to be quite clear that the latter are descended directly from the former. I must therefore 
differ decidedly from Abel, who (1919, pp. 186—189) did not recognize the Catopterids 
as an independent family, but grouped it with the Palaeoniscids (cf. p. 250 above). 
I have already pointed out above (pp. 249—250) that such forms as Boreosomus and 
Acrorhabdus among the Palaeoniscids appear with regard to the anatomy of the head 
to be specialized in the same direction as the Catopterids, while the fins exhibit the 
conditions typical for Palaeoniscids. On the other hand it is also of interest to find that 
such genera among the Catopterids as Catopterus and Dictyopyge show typical Pala- 
eoniscid-like conditions in the lepidotrichia, while Perleidus and Dollopterus are in this 
respect developed like genuine Protospondyli. 
