476 
AVELLBUllN : ON THE GENUS CCELACANTIIUS. 
appearance of an external element." Again, in the author's collection 
there are specimens which to him prove that the bone was external, 
and that it is the maxilla. Of the premaxilla (P.Mx.), its characters 
are somewhat doubtful, as it is not well shown in any specimen, 
the head here being generally seen in a crushed condition. The 
mandible is well seen in many specimens, both in situ and detached. 
Its structure is somewhat complex, and it consists of the following 
parts, viz., the greater portion of each mandibular ramus is formed 
by a long, narrow, articulo-angular element (Art. An.), ornamented 
on its external surface ; its inferior margin is nearly straight, its 
superior arched in advance of the articular facette, behind which 
there is a short extension. A small toothless dentary element (D.) 
meets this in front, reaching to the symphysis, and bounded below 
by a thin infradentary (I.D.). A long, deep, lamina sphenial bone, 
tapering in front, but with a straight dentigerous border in the 
greater part of its length, is opposed to the dentary and articular- 
angular on their inner face, this forming the wall of a vacuity existing 
between the upper portion of the two outer elements. 
The branchio-stegal apparatus is represented by a pair of large 
opercular (Op.) bones, which fill in a more or less triangular space 
between the parietals, the cheek plates, and the shoulder girdle, 
and by two elongated jugular (J.) plates which fill in the space in 
the gular region between the mandibular rami. 
Internal Cranial Anatomy. 
The chondrocranium is extensively ossified, but there is no 
interorbital septum, and the base is formed by a long slender para- 
sphenoid bone, which exhibits a spatulate expansion anteriorily. 
The hyomandibular and pterygo-quadrate arcade are fused into 
a continuous triangular, lamelliform bone on each side, articulating 
with the hinder portion of the cranium above, and is below provided 
with a ginglymoid condyle for the articulation of the mandible. 
The bone terminates in an attenuated angle in front, and its superior 
portion inclines inwards to form the roof of the mouth. This surface 
is finely granular, whilst the outer surface is smooth. In front of 
j-he pterygo-quadrates are a pair of small palatine bones, and in 
