218 report— 1846. 



trosal, 'rocher*.' But the real difficulties which beset the quest of general 

 truths in comparative osteology are such that we may well dispense with any 

 over-statements of the amount of deviation from the cranial archetype which 

 much -modified skulls like those of the anourous batrachia may present. 

 Fortunately the light which the development of such skulls throws upon 

 their mature characters, is aided by the persistent larval stages manifested 

 by the perennibranchiate species. 



In the menopome, for example, the prefrontals remain distinct, both from 

 each other and from the orbitosphenoidsf, their characteristic connections 

 and functions being the same as those of their coalesced homologues in the 

 frog, except that they are notched, instead of being perforated by the olfac- 

 tory nerve, which grooves their inner border, as in the cod and some other 

 fishes. Cuvier just hints at the possibility of his 'os en ceinture' in the frog 

 representing "a la fois le frontal principal et rethmoidej," or as having an 

 equal pretence to one or the other name. 



The suture, however, which marks the limits between the frontal 11 and 

 parietal 7 is persistent in the menopome, and indeed in all batrachians but 

 the anourans ; and even in the very young larva? of these, Cuvier admits 

 (and the observations of M. Duges warrant the admission) " que Ton separe 

 une partie posterieure de forme ronde de l'anterieure qui est allongee" (Ibid. 

 p. 387). The permanently distinct frontals present a similarly elongated form 

 in the urodeles, and are therefore recognized by Cuvier in the salamander, 

 e. g. at c, pi. xxv. fig. ] , op. cit. ; in the newt, pi. xxvi. fig. 6 ; in the menopome, 

 fig. 4 ; in the axolotl, pi. xxvii. fig. 24 ; in the siren, ib. fig. 2 ; and in the ara- 

 phiuma, ib. fig. 6. In all these crania the true frontals are indicated by the 

 same letter c ; in none of them do they close the cranial cavity or bound the 

 orbits anteriorly, or are perforated by the olfactory nerves, or articulate with 

 the vomer below, or perform any of the essential functions, or combine the cha- 

 racteristic connections of the prefrontals of fishes, all of which concur in the 

 ' os en ceinture.' But the frontals do present the chief connections and occupy 

 the relative position of the anterior half of the. bone (11 — 7, fig. 13) which 

 Cuvier calls the parietal in the frog. The evident tendency to coalescence of 

 essentially distinct bones which pervades the skeleton in the adult anourans 

 greatly diminishes the difficulty, through the loss of the suture between the 

 parietal and frontal, of recognizing the homology of the latter bone, which, 

 with that exception, not only repeats the characters of the frontals in fishes, 

 but of those in most tailed batrachians. 



Next, then, with regard to the ethmoid, the second of the two bones to 

 which Cuvier restricts the choice of the homologues of the 'os en ceinture,' 

 no. 14. No name has been applied more vaguely or with a less definite 

 meaning than this same ' ethmoide.' In the sense in which Cuvier would 

 permit its application in the present instance, it is a bone which forms the 



* Op. cit. p. 386. 



f The menopome, which represents a gigantic tadpole of the tailless batrachia, manifests 

 a beautiful conformity to the general type, and well illustrates the real nature of the apparent 

 deviations which take place in the course of the remarkable metamorphoses of the anourans. 

 At first sight the orbitosphenoids seem to be barred out from their normal connection with 

 the frontal by the junction of the parietal with the prefrontal in the menopome, as appears, 

 for example, in the figure given by Cuvier in the ' Ossemens Fossiles,' v, pt. ii. pi. xxvi. fig. 4, 

 where c h divides c from u. Remove, however, the prefrontal h from the parietal c (which 

 may be readily done, the suture, which is not indicated in the figure cited, being persistent), 

 and the anterior and mesial half of the orbitosphenoid («) is then seen extending inwards 

 (mesiad), beneath the parietal and prefrontal, to join a triangular surface formed by a de- 

 scending process from the middle of the outer edge of the frontal. 



J Op. cit. p. 388. 



