178 RAMSAY H. TRAQUAIR 
(7.7.) is clearly shown in several specimens, and are at least five in number on 
each side. Of these, the hindermost is also the largest, and is situated below 
the lower margin of the suboperculum, extending also beyond the posterior 
margin of the principal jugular; the remaining four are placed between the 
last-named plate and the mandible, and diminish in size regularly from behind 
forwards. There is also the clearest possible evidence of a median jugular 
(mj.), of a somewhat oval-acuminate form, placed immediately behind the 
symphysis of the mandible, and overlapping to some extent the anterior 
extremities of the principal jugulars. That the lateral and median jugular 
plates were not noticed by Professor Youne, is clearly due to the more 
imperfect material then at his command. 
CONCLUSION. 
The foregoing investigation into the osteology of the head of Rhizodopsis, 
deficient as it is with regard to the more internally situated parts, nevertheless 
brings out, in a very striking manner, the affinity of that genus to the rhombic- 
scaled Saurodipterini, and supplies further evidence, were that now required, 
of the comparatively small value of the mere external forms of scales as 
indicating the natural affinities of ganoid fishes. 
No one acquainted with the structure of Megalichthys can fail to be struck 
with the extreme resemblance which its cranial osteology bears to that of 
Rhizodopsis, not only in general arrangement but in the shapes of individual 
bones,—a resemblance shared in as well by the teeth with their labyrinthically 
plicated bases, by the shoulder bones, by the fins in their structure and position, 
and by the vertebral column with its ring-shaped centra. Beyond a doubt, the 
affinities of Rhizodopsis are much more with the rhombiferous Saurodipterini 
than with the cycliferous Holoptychiide, although, on account of the form of 
the scales, both Rhizodopsis and Rhizodus were once included in the genus 
Holoptychius. 
Very distinct family characters are, however, presented by the Saurodipte- 
rini in the scales having assumed a sharply rhombic contour, in their free 
surfaces, as well as those of the cranial bones and fin rays, being covered with 
a layer of brilliant ganoine, and in the tendency of many of the bones of the 
head to fusion with each other. In Megalichthys, for example, the mandible 
though closely resembling that of Rhizodopsis in external contour and in the 
form and arrangement of its teeth, has the elements—which in the latter genus I 
have designated as angular, dentary, infradentary, and internal dentary—all 
fused into one piece, an oblique line on the outside of the jaw usually indicating 
the original separation of the dentary. In some Old Red Sandstone Sauro- 
dipterini the original separation of the parietal, sguamosal, and posterior frontal 
