1884.] 5 i 5 [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 parietal bone. Ex- 

 ternal to this and the occipital, and filling the space behind as well as an- 

 terior to the postero-lateral angle of the parietal, 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." f The element anterior to the 

 parietal 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 Lepidosirenidae4 In this family and in the Ceratodontidse these 

 bones are more 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, Stanniusg and Giinther identify these elements with thefrontals 

 in the genera they have described. Huxley | calls them supraorbitals, so 

 that it becomes necessary to name the median posterior element a fronto- 

 parietal, 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. Each 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, PI. I. 



t 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 Wirbeltbiere; Rostock; Erstes Buch, die 

 Fische. 1854, p. 49. 



| Anatomy of Vertebrated Animals, 1871, p. 145. 



