FAUNE DU CALCAIRE CARBONIFERE DE LA BELGIQUE. 
19 
subopêrculûm, but which 1 bave now corne to look upon as interoperculam. It is irregularly four- 
sided; higher than long; its superior and inferior margins, tire former the shorter and oveilapped 
by the lower border of tbe operculum, pass obliquely upwards and backwards; the anterior margin, 
slightly concave, passes downwards and slightly forwards along tbe liyomandibular for tbe greater 
part of the lengbt of that element; the posterior one slightly overlaps the front of tbe clavicle, and 
tbe angle wbicb it forais with tbe inferior rnargin is gently rounded off. The facial bones covering 
tbe cheek in front of these opercular plates are, with the exception of the suborbital mentioned 
above, too mucli injured to admit of any description. Only a portion of the edge of the maxilla (ma?.) 
can be seen forming part of the upper margin of the mouth. The lower jaw is stout, one inch in 
length —- a large portion of its upper margin is split off in front, and unfortunately no teeth can 
be made out, either on it or on the maxilla. Along the ouler aspect of the posterior part of the 
lower jaw, there is a peculiar appearance as if it were covered by a row of oval scales imbricating 
from before backwards; if this be really the case, and the appearence be not due to external 
sculpture, it is a feature, so far as I am aware, unique in the structure of this group of fishes. The 
same scaly appearance is observed in the case of the branchiostegai rays (br.), which, following 
upon the lower margin of the interoperculum, forms a sériés of eight or nine narrow plates extending 
forwards beneath the ramus of the lower jaw; they are elongated at first but get rapidly shorter 
as they proceed from behind forwards. 
Placed behind the posterior margin of the cranial sbield, is a subtrigonal plate (p. ?.), with 
posteriorly directed rounded apex, and which is evidently the homologue of the plate, which, in 
the Palæoniscidæ, I hâve designated post-temporal (supra scaplar, R. Owen). Ils inferior margin 
overlaps, and is articulated with the upper exlremily of the supra-clavicular (s. cl.) a bone of larger 
size, and more elongated forai, whose direction is from above downwards and slightly backwards, 
and whose bluntly pointed inferior extremity overlaps the upper end of the clavicle (cl). Thelatter 
is a bone of great strength passing downwards and also somewhat forwards, slightly overlapped by 
tbe interoperculum and the upper members of the branchiostegai sériés. In form it is gently sickle— 
shaped with forwardly directed concavity; the apex is concealed by the overlapping extremity of 
the supra-clavicular; the inferior extremity is somewhat expanded. A small portion of the interoper- 
culum being broken away just in front of the upper extremity of the clavicle, part of the anterior 
