1876.] PROF. T. H. HUXLEY ON CERATODUS FORSTERI. 39 



The branchial apparatus of Lepidosiren differs from that of Cera- 

 todus mainly in the greater number of complete branchial arches. 



It can hardly be doubted that the bone D of Ceratodus is repre- 

 sented, though incompletely, by the supraorbital of Lepidosiren, while 

 the bony nasal shield of the latter corresponds very closely with the 

 anterior median bone {A) of Ceratodus. The posterior boundary of 

 the bone, however, lies further back in Ceratodus than it does in 

 Lepidosiren. The argument of Dr. Gilnther that the posterior 

 median bone (B) in Ckratodus is not the homologue of the parieto- 

 frontal of Lepidosiren, because it lies above the muscles, while the 

 latter is situated beneath them, is weighty against the identification 

 of the bones in question ; and, in other respects, the parieto-fiontal 

 of Lepidosiren is very unlike the " scleroparietal " of Ceratodus. 



When the comparison of the cranial and facial bonas of Ceratodus 

 with those of the Vertebrata is extended beyond the limits of the 

 Dipnoi, the determination of their homologues is beset with many 

 difficulties. Polypterus has an anterior and a posterior median 

 shield in the roof of the skull, which at first seem to correspond with 

 those of Ceratodus ; these shields are each formed by the union of 

 two bones, which are evidently comparable to the frontals and parietals 

 of the higher Vertebrata, while the frontals unite with a pair of broad 

 nasals which cover the olfactory sacs. The apices of the posterior 

 triangular edges of these bones reach back to near the level of the 

 middle of the orbits ; and the frontal bones are continued forwards 

 on each side of them. Between the two nasal bones there is a median 

 ossification which lies upon the mesethmoidal cartilage and spreads 

 out in front, ending by a broad edge which articulates with the prae- 

 maxillse. 



The median bone, the piscine "ethmoid," occupies the same 

 position as the anterior median bone of Ceratodus would do if the 

 ethmoidal region were reduced to the proportions it has in Poly- 

 pterus. Therefore, from this point of view, the determination of the 

 bone as "ethmoid " by Dr. Giinther seems fully justifiable ; and the 

 inner lateral and the median posterior bones would seem to represent 

 the frontal and parietal bones of Polypterus. 



On the other hand, the many points of resemblance between Cera- 

 todus and the Amphibia suggest the comparison of the anterior and 

 posterior median bone to the frontals of Menobranchus, and of the 

 inner lateral bones to the parietals of this Amphibian. The forward 

 extensions of the latter, at the sides of the frontals, are especially 

 noticeable in comparison with the anterior extremities of the inner 

 lateral bones of Ceratodus. On the whole, I am inclined to think 

 that Polypterusis the better guide in the interpretation of the cranial 

 bones of Ceratodus, though the difference between the bones of 

 Ceratodus and those of the Crossopterygian ganoids, all of which are 

 readily reducible to the Polypterine type, is very considerable. 



In other respects the skull of Ceratodus finds its closest parallel 

 among the Amphibia, especially such Urodela as Menobranchus*, 

 and the Anura in their tadpole state. 



* See P. Z. S. March 17, 1874. 



