STRUCTURE OF CERTAIN PALEOZOIC DIPNOI. 207 
surface. Here the space between the rami of the lower jaw is mainly occupied 
by the two pairs of large gular plates already described ; but in advance of 
these are three other small bones, two of which are ob^dously a pair meeting 
in the middle line, whilst the other more lateral element which on the right 
side separates them from the anterior end of the large giilar is concealed on 
the left side by a forward displacement of that bone. At the spot where the 
four large gulars meet is a small median lozenge-shaped element, already 
figured, as were the large gulars, by Pander. 
No specimens known to us show the structure of the shoulder-girdle 
completely. 
There are a scale-like post-temporal and supra-cleithrum connecting the 
upper end of the cleithrum with the tabular. The cleithrum is an elongated, 
narrow bone with a I'ecess on its outer surface for the reception of the hinder 
edge of the operculum. It is so rigidly attached to the clavicle, that that 
bone usually retains its natural position and shape in the fossils, having 
resisted the crushing which disarticulates most of the other bones. 
There is some evidence (Peach Coll., No. 35, Royal Scottish Museum) that 
this attachment is effected by a sj^ecial downwardly-projecting process on the 
inner surface of the cleithrum, which is received in a recess in the clavicle : 
that, in fact, the structure here is exactly as in Sagenodus. 
The clavicle is a massive bone turning inward and forward onto the ventral 
surface, and with its fellow filling up the triangular space between the 
principiil gulars. 
There is some evidence of a large scale in the position of an interclavicle. 
DiPTERUS PLATYCEPHALUS, Ag. 
We are unable to add much to the existing knowledge of the skull of 
D. jilatycephalus. No specimen known to us shows the circumorbitals in 
intelligible form, and none gives an altogether satisl'actorj^ view of the 
opercular region. No. 1059, Hugh Miller Collection, Edinburgh, and 
Pander's figures, Taf. 3, fi'g. 17, and Taf. 4, fig. 26, show only three bones 
on eacli side — an operculum, sub-operculum, and a gular ; no other specimens 
show additional elements, and it is thus possible that the apparatus was far 
more reduced than \n D. valenciennesi, although the material does not admit 
of definite statements. 
The structure of the lower jaw (cf. fig. 34, p. 208) is perfectly shown by 
No. L. 10858 of the Manchester Museum. The general features of the 
morphology were accurately figured by Traquair, but that author was not 
acquainted with certain very important characters, vividly shown in our 
specimen. 
The dentai'ies are small elements forming a rim to the anterior end of the 
mandible ; the two bones are indistinguishably fused. In section their outer 
surface forms nearly three quadrants of a circle passing smoothly from the 
LINN. JOURN.— ZOOLOGYj VOL. XXXV. 14 
