170 RAMSAY H. TRAQUAIR ON 
‘ post-frontal” and ‘squamosal.” Now, as the bones of the skull of Teleostean 
fishes, known in the Cuvierian system of nomenclature as “ post-frontal” and 
“mastoid,” are ossifications in the periotic portion of the primoidial cranium 
(sphenotic and pterotic of Parker), and as the disputed bones in the cranial 
buckler of the Crossopterygian Ganoids above referred to are evidently dermal 
in their nature, the latter may be considered as really partaking more of 
the nature of the ossa intercalaria in Polypterus. But as to their being 
considered exactly the equivalents of those little plates in Polypterus, there are 
some pretty serious, and to my mind fatal objections. They are firmly united 
by suture to the outer margin of each parietal, with which they form an integral 
part of the cranial buckler. In the Lepidosteoid Ganoids (Lepidosteus, 
Lepidotus, &c.), there is, external to each parietal, a plate (sgwamosal) evidently 
corresponding to the posterior of the two in Rhizodopsis, &c., and which no 
one has ever thought of considering homologous with the Polypterine inter- 
calaries. The same plate is found in Ama, and there is in addition another 
smaller one in front of it corresponding to the anterior of the two in Rhizodopsis, 
but which, from the relatively greater shortness of the parietal, and the corre- 
sponding greater extension backwards of the frontal, comes to lie external to 
the posterior part of the outer margin of the latter. In the Paleeoniscidee there 
are also two corresponding plates, but the anterior of these, which I have 
lettered as post-frontal in my memoir on the structure of this family,* is placed 
relatively to the frontal still further forwards, owing to the greater proportional 
length of the squamosal behind it. In Acipenser there is also, external to 
the plates which seem to represent the parietals and frontals of other fishes, a 
chain of two or more smaller plates, which apparently represent those in 
question, and which, firmly articulated with the others covering the cranial 
cartilage, lie zms¢de the position of the spiracle. There is no spiracle in 
Lepidosteus or Amia, and no evidence of it in the Paloniscide, or in either the 
Rhombo- or Cyclodipterine Crossopterygii, but in Polypterus there is, and the 
chain of intercalary ossicles, loosely articulated to the margin of the cranial shield, 
lies external to the spiracular slit, which passes down between two of them 
and the side of the cranium proper. It therefore seems to me inappropriate 
to consider the bones p.f. and sg. of the cranial shield of Rhizodopsis and allied 
forms to be the homologues of the intercalary ossicles in Polypterus, and better 
to follow Professor Huxtey in designating them respectively as post-/rontal and 
squamosal, always bearing in mind, however, that the former has nothing to 
do with the post-frontal of Cuvier, for which it is better to adopt the term 
“ sphenotic”” as proposed by Parker. In Ama, in fact, a well-developed 
sphenotic coexists with the more superficial plate to which I have referred as 
“ post-frontal.” 
* “Carboniferous Ganoids,” Paleontographical Society, 1877. 
