40 PROF. T. H. HUXLEY ON CERATODTJS FORSTERI. [Jail. 4, 



I have already indicated the chief points of resemblance to the 

 amphibian skull, and need not recapitulate them here. The most 

 important feature is the manner in which the mandibular arch is 

 connected with the skull. 



The part of the palato-quadrate cartilage which is united with the 

 skull, between the exits of the fifth and second nerves, answers to 

 the " pedicle of the suspensorium " of the amphibian, while its 

 backward and upward continuation onto the periotic cartilage corre- 

 sponds with the otic process. As in the Amphibia and in the higher 

 Vertebrata, the mandibular arch is thus attached directly to the skull 

 by that part of its own substance which constitutes the suspensorium. 

 It may thus be said to be autostylic. 



Among fishes, the only groups which possess an autostylic skull, 

 or in which the dorsal end of the mandibular arch is continuous with 

 the cartilage of the brain-case, are the Chimseroids and the Marsipo- 

 branchii. 



In Clnmcera, the general form and connexions of the palato-quadrate 

 cartilage are the same as in Ceratodvs ; but it differs from that of 

 Ceratodus as that of the tadpole differs froin that of a young Frog, 

 or as that of Metwbranc/ms differs from that of Menopoma; that is 

 to say, the articular condyle is situated far more forward, and the 

 gape is, in consequence, relatively shorter in the former than in the 

 latter. There are the same large olfactory capsules in both cases. 

 In Cht'mcera, however, these project beyond the termination of the 

 ethmoidal cartilage, while in Ceratodus the latter projects beyond 

 the olfactory capsules, which are more lateral in position, more elon- 

 gated, and, in accordance with the general form of the head, much 

 more depressed. 



Just as in Ceratodvs, the palato-quadrate cartilage of Chimcpra 

 bears two teeth marked with radiating ridges, while two others, the 

 vomerine teeth, are supported by the ethmoidal cartilage in front of 

 these ; and in both cases there is a tooth with radiating ridges on 

 its surface in each ramus of the mandible. 



In the disajipearance of the notochord and the articulation of the 

 skull with the anterior coalesced vertebrae, the skull of CJdmcEra pre- 

 sents a higlier degree of differentiation than that of Ceratodus ; while 

 it is needless to speak of such aberrant peculiarities as its supracerebral 

 iuterorbital septum, or the vast crest into which the skull is raised 

 above the anterior part of the brain-cavity. In other respects, how- 

 ever, as in the inclination of the axis of the susj)ensorium already 

 noted, the skull of Chimcera presents lower characters than that of 

 Ceratodus. Among these may be reckoned the great size of the 

 upper and lower labial cartilages and the condition of the hyoidean 

 arch, which, except in size and some peculiarities of form, is altogether 

 similar to the four branchial arches which follow it. Like them, it 

 terminates, dorsally, in a flat, expanded, triangular piece, which is 

 connected with the superjacent floor of the skull by muscles and 

 ligaments, but by no direct articulation. The dorsal pieces of the 

 succeeding brancliial arches have the same form and attachments, 

 and unite with the ventral segments at a sharp angle. These angles 



