198 PROF. T. H. HUXLEY ON MENOBRANCHUS. [Mar. I", 



On the other hand, the latter is much like that of a Lamprey, if 

 we leave the ossifications of the Menobranchus skull, and the accessory- 

 cartilages of the Petromyzon, out of consideration. And this fact, 

 taken together with the curious resemblances in development between 

 the Lampreys and the Amphibia (which are much closer than those 

 between any of the higher Fishes and the Amphibia) *, suggest to 

 my mind the supposition that, in the series of modifications by which 

 the Marsipobranch type has been converted into that of the higher 

 fishes, the most important terms must have been forms intermediate 

 in character between the Dipnoi and the Marsipobranchs. The 

 skeleton of such a fish as Ceratodus, if it had a Menobranchus-like 

 choudrocranium, would approach that of the Lampreys more than 

 that of any fish known at present ; and it is not difficult to imagine 

 the steps by which such a fish might be built up upon the " lines " 

 of a Lamprey. 



The bearing of the structure of the chondrocranium in Menobran- 

 chus, the larval Triton and Siredon upon the theory of the skull is 

 obvious. 



It is plain that three morphologically distinct elements enter into 

 the composition of the cranium in these animals : — 



1. The parachordal elements or "investing masses" of Rathke, 

 which stand in the same relation to the notochord, in the skull, as 

 the formative tissue out of which the bodies of the vertebrae are 

 developed, in the spinal column. 



2. The pleural elements or visceral arches, which are divisible into 

 trabecular, mandibular, hyoidean, and branchial. 



3. The paraneural elements or capsules of some of the organs of 

 the higher senses. 



The brain-case is a complex structure, formed by the coalescence 

 of elements belonging to all three classes ; the face, by the meta- 

 morphosis of visceral arches only. 



The occipital portion of the chondrocranium, which lies behind 

 the auditory capsules, and which, by its ossification, gives rise to the 

 proper exoccipitals in Amphibia, and, in addition, to the basiocci- 

 pital and supraoccipital in Osseous Fishes and the higher Vertebrata, 

 appears in all cases to result from the metamorphosis and ossification 

 of parachordal elements. 



On the other hand, that portion of the chondrocranium which lies 

 in front of the auditory capsules is, in the Amphibia, formed by the 

 coalescence and metamorphosis of the trabecule. It is by their 

 vertical growth that the relatively high lateral walls of the cranium 

 of the Frog are formed ; it is by their coalescence in the ventral 

 median line that the pituitary space becomes completely floored with 

 cartilage in the same animal ; and it is by the outgrowth of alary 

 processes from the coalesced internasal, or mesethmoidal, portions of 

 the trabeculae, that the roof and floor of the Frog's nasal capsules are 

 produced. 



Menobranchus is exceptional among the Amphibia in presenting no 

 ossification of the substance of the trabeculse in front of the pro-otic. 

 * Unfortunately we know nothing of the development of the Dipnoi. 



