STRUCTURE OF CERTAIN PALEOZOIC DIPNOI. 201 
ceratohyals are well shown ; they exactly resemble the corresponding bone& 
in Ceratodus. 
The shoulder-girdle is remarkable for the very small relative size of the 
clavicle. 
The cleithrum is a large bone of much greater width than is shown in the 
figures, where jjart of it is hidden by matrix in each case. The upper end is 
comparatively narrow, and is seen to be covered by a very badly preserved 
bone, no doubt the supra-cleithrum. The inner surface is concave, the outer 
flat with a turned-in anterior margin ; it has a depressed area for the 
hinder edge of the operculum. The lower end of the cleithrum turns 
forward and is largely covered by the sub-operculum, appearing behind that 
bone only as a narrow strip. 
The clavicle is displaced and shown only from its outer surface. It is- 
extremely short, attached to the cleithrum by a deeply recessed triangular 
area on its outer surface, and widening both ways ventrally. 
The neural arches are thoroughly ossified anteriorly, although there are no- 
ossifications in the notochordal region. The anterior neural spines are 
separately ossified from their arches. The ribs are slender, well curved and 
with slightly thickened heads. 
The structure of the median fins is already familiar; there is a continuous 
fin agreeing exactly in structure and distribution with that of Ceratodus. 
The pectoral fins are shown by the Edinburgh specimen to be large, typical 
biserial archipterygia, the Etxis being entirely uuossified, whilst the radials of 
both series have short, hollow ossifications. There are very well-developed 
camptotrichia which do not extend in to overlap the ossified radials. 
The structure of the pelvic fins is not so clearly shown, but they obviously 
agree in general with the pectoral fins, and are nearly as large. 
The scales are not very well shown; they are thin, of considerable size, and 
marked with very delicate concentric rings of growth. It is impossible to 
say how far forward they extended. 
ConcJiopoma must have been extremely like Ceratodus in proportions and 
general build, although the head may have been sligbtlj^ lower posteriorly. 
Uronemus splendens (Traq.). 
The genus Uronemus was founded by Agassiz for some small fish from 
the Burdiehouse Limestone, which have a continuous median fin and an 
apparently diphycercal tail. To this genus Dr. Traquair I'eferred some 
remains from the Lower Carboniferous, No. 2 Ironstone, Loanhead, Mid- 
lothian, which show many details of skull structure. Beyond stating that 
these specimens show a skull-roof like that of Ctenodus and prove that the 
pterygoids and " splenials " bear a single series of large, compressed, low, 
conical teeth in addition to a granulation of small denticles, there being no 
