62 
AVELLBURN : ON THE GENUS MEGALICHTHYS, AGASSIZ. 
ethmoidal (E.), with the Nasals (N.) on each side, and more 
external still the bone probably represents the Pre- or Anterior 
frontals (A.F.). The nasals were perforated by and contained 
the olfactory organs, and the nares (Nsl.) were anteriorly placed 
on each side of the snout, as is the case in Osteolepis and in the 
recent fish Polypterus (see PI, XV., Fig. B). 
Behind the cranial shield, in the occipital region, are three 
bones, one median and two lateral. The central one is in the 
form of a narrow isosceles triangle, and the lateral ones form a 
pair on each side of it. These bones are regarded as the supra 
temporals by Dr. Traquair, Mr. Smith Woodward, and others. 
Side of the Head. (PI. XIII. and XV.)— This region is 
entirely covered with loose dermal plates. 
The Orbit is anteriorly placed, being situated at the junction 
of the anterior and middle thirds of the head. Above it is 
bounded by the frontal bones, in front by a bone (A.O.) which 
probably represents the Anterior Orbital of Polypte^-us, below by 
an elongated triangular bone (S.O.), the Sub-Orbital, which rests 
on the anterior portion of the upper edge of the maxilla (Mx.) 
below, and extends from the pre-maxilla in front to a third bone 
(S.O^)., which is triangular in shape, and forms the posterior 
inferior boundary of the orbit. This bone is the supra-orbital 
of Prof. Huxley. Behind the orbit is a square - shaped bone 
(P.O.), the post orbital. 
Behind the orbital region is a large plate (X) which is 
somewhat oval and obliquely placed, it covers a large portion 
of the cheek, and is bounded in front by the post- and supra- 
orbitals, behind by an elongate bone (P. Op.), above by parts of 
the posterior frontals and synamosals, and below by another plate 
(X^) which is somewhat rhomboidal in shape and fills the space 
between the larger cheek plate (X) above, the articular extremity 
of the mandible below, the hinder border of the maxilla in front, 
and the lower third of the anterior border of the bone (P. Op.) 
behind. This latter bone (P. Op.) may be considered as the 
preoperculum. It is a narrow, elongated, somewhat arched bone, 
