of Edinburgh, Session 1876 - 77 . 
445 
portion forms the depressed rounded snout, the front margin of which 
above the mouth, is formed by two small dentigerous praemaxillary 
bones entirely differing in form from the bones described as such by 
Messrs Hancock and Atthey. The gape is wide, the kyomandibular 
bone being slightly inclined backwards; there is apparently no 
symplectic. The form of the maxilla is well known ; the structure 
of the mandible is exceedingly complex. Its dentary element is 
found to correspond with the bone interpreted by Messrs Hancock 
and Atthey as “ prcemaxillaf but turned in a contrary direction, 
i.e., with its toothed margin upwards instead of downwards as 
supposed by them ; it bears the anterior laniary tooth and the 
outer row of small teeth. The laniaries posterior to the front one 
are borne on separate internal dentary ossicles, the presence of which 
is clearly proved by a portion of a jaw of a large specimen of 
Rhizodopsis in the Edinburgh Museum. The external aspect of the 
lower jaw is completed by at least two infra-dentary plates situated 
below the inferior margin of the dentary, behind which there is 
another covering externally the articular and angular region of the 
jaw, and which is probably the equivalent of the angular of other 
ganoids. On the internal aspect of the mandible there is, besides 
the internal dentary or laniary ossicles, a well-marked splenial. 
The orbit is very anteriorly placed ; as in Megalichthys , three plates 
cover the cheek behind the suborbitals, corresponding to the single 
large one in Osteolepis. The author does not find Professor Young’s 
statements that the jugular plates of Rhizodopsis are “ in two pairs, 
principal and posterior,” and that there is “ no trace of median or 
of lateral plates,” corroborated by the specimens under examination. 
On the other hand, he finds one pair of large principal jugulars 
with at least four lateral ones on each side, as well as very distinct 
evidence of the presence of a median plate behind the symphysis of 
the jaw. The shoulder girdle is provided with well-developed 
infraclavicular plates. 
The lower jaw of Rhizodus Hibberti is found to have essentially 
the same structure as that of Rhizodopsis. The dentary element 
is of the same form, and bears the anterior large laniary tooth and 
the outer range of small teeth, the posterior laniaries being borne 
on separate ossicles, which are sometimes found entirely detached. 
3 N 
VOL. IX. 
