1884.] 575 [Cope. 
Returning to the superior face of the cranium, we observe that the 
exoccipital elements form a wedge-shaped body, divided on the middle 
line by suture, with the apex forwards. Traces of this division are figured 
by Gegenbaur as present in Heptanchus.* Anterior to this the middle 
of the cranial roof is apparently occupied by another triangular bone with 
the base posterior and the apex anterior, and concealed beneath the free 
extremity of the element in front of it, The lateral sutures only are dis- 
tinguishable, appearing as grooves (fig. 2). This is the pariétal bone. Hx- 
ternal to this and the occipital, and filling the space behind as well as an- 
terior to the postero-lateral angle of the pariétal, is the element which is 
produced outwards and backwards as already described. Were I describ- 
ing a true fish, this bone might be intercalare (epiotic) or pterotic. Perhaps 
it is both combined, or it may be the cartilage bone called by Giinther, 
in Ceratodus, the ‘tympanic lamina.’’?+ The element anterior to the 
pariétal is the cartilaginous representative of the frontal, and the fact 
that it terminates posteriorly in two free processes is significant of the 
true homology of the bones which terminate in like manner in the crania 
of the Lepidosirenidx.{ In this family and in the Ceratodontide these 
bones are inore or less separated on the middle line by the median pos- 
terior element. In Ceratodus the separation is wide ; in Lepidosiren the 
interval is uninterrupted, but narrow in front. In Protopterus these 
elements are in contact on the middle line, but diverge posteriorly. 
Bischoff, Stannius$ and Giinther identify these elements with the frontals 
in the genera they have described. Huxley] calls them supraorbitals, so 
that it becomes necessary to name the median posterior element a fronto- 
pariétal, as a combination of two bones usually found distinct in fishes. 
The furcate structure of the frontal cartilage in Didymodus goes to show 
that the identification by Bischoff and Giinther is the correct one. There 
are also in this genus distinct paired membrane bones which do not take 
part in the bifurcation in question, and which appear to represent the 
frontals of Ceratodus. Hach of these is a flat, subcrescentic supraorbital 
plate, which has a concave superciliary border. It is separated by a con- 
siderable interval from its fellow ‘of the opposite side. Its anterior 
extremity is notched by a fossa which I suppose to represent the ante- 
terior (posterior in position) nostril. The ? frontal of the right side is dis- 
placed, and appears as a lamina lying on the frontal cartilage, showing 
that it is a membrane bone. From its relation to the nostril the question 
arises, whether it be not the homologue of the nasal. 
For hyomandibular bone, palatopterygoid arch, and mandibular arch, 
we have to rely principally on one specimen. On one of the skulls, two 
* Ueber den Bau des Schedels der Selachier, 1872, Pl. I. 
+ Philosophical Transactions of the Royal Society, 1871, p: 511, indicated on 
the plates by the letter d. 
}$ Lepidosiren paradoxa by Bischoff, Prof. in Heidelberg ; Leipsic, 1840, 
@ Handbuch der Anatomie der Wirbelthiere; Rostock ; Erstes Buch, die 
Fische, 1854, p. 49. 
| Anatomy of Vertebrated Animals, 1871, p. 145. 
PROC. AMER. PHILOS. SOC, xxt. 116. 8u. PRINTED JULY 1, 1884. 
