THE CRANIAL OSTEOLOGY OF RHIZODOPSIS. 177 
Polypterus, it is, I think, pretty evident that the bone p.op., together with the 
one 2 in Lhizodopsis, corresponds to the large cheek plate in the former genus, 
considered by Acassiz to consist of the equivalents of the cheek cuirass in 
Lepidosteus united with the preeoperculum, while the lower one (2’) apparently 
corresponds to the posterior of the two small plates, which in Polypterus are 
placed below the inferior margin of the large one and behind the maxilla. The 
bone p.op. in Rhizodopsis may then be considered as the prwoperculum, the 
~ two others, x and a’, as equivalent to the cheek cuirass in Lepidosteus, or to 
the posterior set of sub-orbitals in other Lepidosteids (e.g., Lepidotus), and in 
the Paleoniscide. 
In front of the bone 2, and above the maxilla, there are in some specimens 
evident enough remains of the proper sub-orbitals, which seem to have cor- 
responded in number and position pretty closely to those in Osteolepis. Two 
of them (s.o. fig. 2)), corresponding respectively to the posterior-inferior and 
anterior-inferior parts of the boundary of the orbit, are clearly seen in many 
specimens, but the unfortunate manner in which the heads are crushed renders 
any further description hardly possible. 
The space between the right and left mandibular rami is occupied by a set 
of jugular plates. Professor YouNG 
has described these as consisting of 
“two pairs, principal and posterior,” 
and has also stated that there is ‘no 
trace of median or lateral plates.” * 
The specimens before me, however, do 
not corroborate the views above quoted. 
I find two principal jugulars (7. figs. 2 
and 3) occupying almost the whole of 
the space. Each of these is of the 
usual oblong shape, and broader behind 
than in front. The short and rounded 
posterior margin passes uninterruptedly 
into the internal one, which is more 
convex than the external for the Fic. 3.—Under Surface of the Head of Rhizodopsis sawroides. 
greater part of its length ; near the mn. mandible pe principal jugular ; 7.7. lateral jugular ; 
; . m.j. median jugular ; s.op. suboperculum, 
front, however, the internal and ex- 
ternal margins converge and meet in an acute angle. What Professor Youne 
means by a “posterior” jugular I am unable to determine, unless he 
has mistaken for such a plate the broad infra-clavicular element of the 
shoulder girdle, which, as in the recent Polypterus, is overlapped by the 
posterior margin of the principal jugular. The presence of lateral jugulars 
2 Op. cil.. 9p. 596, 
