I IO 
ERIK A : SON STENSIO 
Squamation. 
As- far as one can judge from the few scales present, these were oval and rather 
low (PI. 18, fig. i). Both their anterior and posterior margins are .well rounded, the upper 
and lower ones, on the other hand, are fairly straight or at any rate only weakly convex. 
The exposed part of the lateral surface is somewhat large. The sculpture consists of a 
rather large number of sharp, short striae, which are situated close together on the 
anterior middle part of the exposed surface and run in an antero-caudal direction, fairly 
parallel to each other. In a zone along the dorsal, posterior and ventral margins they 
are sparse and are so arranged that they diverge posteriorly in about the same way 
as in Libys. 
* 
Geological occurrence and localities. — M. lobatus occurs rather infrequently 
at the fish horizon. So far it has only been found at Mt Andersson and Mt Wallenberg. 
Mylacanthus spinosus, n. sp. 
(PI. 19, fig. 3 ; PI. 20, fig. 2) 
There are two opercula (P. 687, P. 70 3 ) of a second species, called by me M. spinosus. 
A specimen with remains of a jugular plate and some of the bones of the mandibula 
(P. 215) probably belongs to this species as well. 
This species has certainly been at least about as large as M. lobatus. 
The two opercula are large. The best preserved one (P. 703) is about 9 cm high 
and almost 6 cm broad (PL 20, fig. 2). In general shape it resembles that of A. robusta 
very closely. Thus one can distinguish an anterior, a postero-ventral and a dorsal 
margin. The last-mentioned of these is, however, not preserved. The anterior margin is 
slightly concave in its upper part, slightly convex at the lower part; the postero-ventral 
margin, on the other hand, is quite the reverse, so that its upper part is convex and its lower 
part chiefly concave.lt is also noteworthy about this margin that all the large sinus of the 
sort described in M. lobatus are quite absent. Instead, however, it is furnished with a large 
number of fine, pointed spines in the way shown in PL 20, fig. 2. The antero-dorsal corner 
seems to have been fairly distinctly developed and almost right-angled; the ventral one is 
rounded, but has a more prominent angle than in the Axelia species and M. lobatus. The 
sculpture resembles in all essentials that of the latter species, but has perhaps been some¬ 
what finer. 
The specimen P. 215 (PL 19, fig. 3 ), on which parts of the supraangulo-angular 
(Sang, ang), dental (De), splenial (Spl) and jugular plates '(G) are to be seen, differs from 
M. lobatus chiefly by having a weaker and finer sculpture on the supraangulo-angular, 
and a considerably stronger development of the pores of the mandibular sensory canal. 
* 
Remarks. — It is not yet possible to decide with certainty whether the specimen 
P. 215 belong to M. spinosus. Accordingly this species can be considered as distinguished 
only by the spiny postero-ventral margin of the opercular, a character which is sufficient, 
however, to keep it apart from M. lobatus, to which it is rather closely related otherwise 
with regard to sculpture and probably also in other respects. 
