FAUNE DU CALCAIRE CARBONIFERE DE LA BELGIQUE. 
19 
suboperculum, bat which I hâve now corne lo look upon as inleroperculam. It is irregularly four- 
sided; higher than long; its superior and inferior margins, lhe former the shorter and overlapped 
by lhe Iower border of the operculum, pass obliquely upwards and backwards; the anterior margin,, 
slightly concave, passes downwards and slightly forwards along the hyomandibular for lhe grcater 
part of the lenght of that élément; the posterior one slightly overlaps the front of the clavicle, and 
the angle which it forms with the inferior margin is gently rounded off. The facial bones covering 
the cheek in front of these opercular plates are, with the exception of lhe suborbital mentioned 
above, too much injured to admit of any description. Only a portion of the edge of the maxilla (mx.) 
can be scen forming part of the upper margin of the mouth. The Iower 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 ont, either on it or on the maxilla. Along the outer aspect of lhe posterior part of the 
Iower 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 lhe structure of this group of fishes. The 
same scaly appearance is observed in the case of the branchiostegal rays ( br.), which, following 
upon the Iower margin of the interoperculum, forms a sériés of eight or nine narrow plates extending 
forwards beneath the ramus of the Iower jaw; they are elongated at fi est but get rapidly shorter 
as they proceed from behind forwards. 
Placed behind the posterior margin of lhe cranial shield, is a suhtrigonal plate (p. /.), with 
posteriorly directed rounded apex, and which is evidently lhe homologue of the plate, which, in 
the Palæonjscidæ, I hâve designated post-temporal (supra scaplar, R. Owen). Ils inferior margin 
overlaps, and is articulated with the upper extremity of the supra-clavicular (s. cl.) a bone of larger 
size, and more elongated form, whose direction is from above downwards and slightly backwards, 
and whose bluntly pointed inferior extremity overlaps the upper end of lhe clavicle (cl.). Tbelafter 
is a bone of great strenglh passing downwards and also somewhat forwards, slightly overlapped by 
the interoperculum and the upper members of the branchiostegal sériés. In form it is gently siclde- 
shaped with forwardh directed concavity; the apex is concealed by the overlapping extremity of 
tiie supra-clavicular; lhe 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 
