Proceedings of the Scientific Meetings of the Zoological Society of London 1876

Page 72

1876.] PROF. T. H. HUXLEY ON CERATODUS FORSTERI. 39 The branchial apparatus of Lepidosiren differs from that of Cera­todus mainly in the greater number of complete branchial arches. It can hardly be doubted that the bone D of Ceratodus is repre­sented, 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. Gunther that the posterior median bone (B) in Ceratodus is not the homologue of the parieto­frontal 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-frontal of Lepidosiren is very unlike the " scleroparietal" of Ceratodus. When the comparison of the cranial and facial bones 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 prae­maxillae. 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 Poly­pterus. Therefore, from this point of view, the determination of the bone as "ethmoid" by Dr. Gunther 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 Cera­todus 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 Polypterus is 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 Cerotodus 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.