128 
ERIK A: SON STENSIO 
The three genera Axelia, Mylacanthus and Scleracanthus differ from the usual type 
and are distinguished by a dentition adapted for crushing. In these forms the parasphenoid, 
the plates on the copula, the pterygoid, the coronoid and the intercoronoideo-prearticular 
are furnished with low, often even flattened, spheroidal teeth, which greatly resemble 
those of Colobodus in their shape and sculpture. Those on the parasphenoid and the 
supracopular plates are largest and most strongly developed. The conditions of the 
maxillary and the precoronoids with regard to dentition are not yet known. Te teeth 
of the branchial arches are, as usual, small and fused with their bases into plates as 
in the other forms. In most cases they are spheroid in shape. 
The sensory canals of the head. — The infraorbital canal, as I have taken it 
in agreement with the definition of Ewart (1892), Ewart and Mitchell (1892) and Cole 
(1896, 1898) etc. may have its posterior end on the boundary between the supratemporo- 
extrascapular and the parieto-intertemporal. It passes in a forward direction throug'h 
the latter bone, reaches the fronto-dermosphenotic, which it pierces in the dermosphenotic 
component. There it turns off laterally and then enters the postorbital on the cheek. It 
penetrates this bone near the anterior margin, after which it passes off downwards and 
forwards in the lacryma-jugal under the entrance to the orbit. Anterior of the lacrymo- 
jugal it has continued ventrally of the nasal apertures, forward to the snout, where, 
situated in the rostralS, it has formed a commissure with the corresponding canal of 
the opposite side. In the fronto-dermosphenotic it anastomoses with the supraorbital canal. 
The supraorbital canal continues from this anastomosis with the infraorbital canal 
in the dermosphenotic component of the fronto-dermosphenotic fairly straight forward and 
after a short distance arrives between the supraorbitals and the lateral margin of the 
frontal component of the fronto-dermosphenotic. In this position it continues along to 
the ethmoidal region, where it passes through the nasal part of the nasalo-antorbital 
dorsally (medially) of the nasal apertures. In front of this last-mentioned bone it enters 
into the lateral rostral plate, bending at the same time first downwards and then 
backwards. It is uncertain whether it anastomosed there with the infraorbital canal. 
In the posterior part of the lacrymo-jugal ( Undina, Libys, Macropoma, etc.) or 
immediately dorsally of this (Wimania, Sassenia) a canal issues in a backward and 
downward arch from the postorbital section of the infraorbital canal. This, for which I 
have adopted the term jugal canal ofMoomE (1908, 1915 a), and which seems to correspond 
in all essentials to the similary situated one in the Stegocephali, runs first through the 
squamoso-preopercular to the lower margin not far from the posterior corner, after 
which it presumably continued in a ventral direction through the posterior part of the 
preoperculo-quadratojugal. It is impossible to decide whether it anastomosed with the 
mandibular canal. 
The mandibular canal penetrates the supraangulo-angular and the splenial and 
extends almost to the symphyse between the two mandibulae. 
The most anterior part of the main lateral line on the head extends, according to 
the definition I have followed for the boundaries of the infraorbital canal, along through 
the supratemporo-extrascapular. Close to or in this bone there issues from it the supra- 
temporal commissure that pierces the extrascapular row of plates. 
It has not yet been possible to observe «pit lines* either on the cranial roof or the cheek. 
