18/6.] PROF. T. H. HUXLEY ON CERATODUS FORSTERI. 45 



spiracle, in correspondence with its homologue the tympano-eusta- 

 chian passage. The dorsal and posterior edge of this process no less 

 clearly corresponds with the spiracular cartilage in Cestracion, other- 

 wise absent in Notidanus. The hyoidean arch is attached to the 

 periotic region of the skull. It is very slender ; and though closely 

 bound by ligament to the mandibular arch, close to the articulation 

 of the mandible, it can contribute little or nothing to the support of 

 the latter. Remove the great otic process of the palato-quadrate 

 arch of Notidanus, and bring its quadrate end further forward, and 

 the result would be a typically amphistylic skull, such as exists in 

 the larval Siredon and Triton*. 



The relations of the skulls of Notidanus, Cestracion, and Cera- 

 todus may be thus expressed : — 



Notidanus most nearly approaches the amphistylic skull, such as 

 exists in all the autostylic Vertebrata in the embryonic state ; but 

 it is considerably altered by the development of a great otic process 

 from the mandibular arch. 



In Cestracion the palato-quadrate has become massive, and, in the 

 region of the pedicle, is firmly united with the skull, while the otic 

 process is a separate cartilage, connected only by ligament with the 

 postorbital process above and with the palato-quadrate below. 



In Ceratodus the palato-quadrate has coalesced with the skull both 

 by its pedicle and by its otic process ; and the same change occurs in 

 the autostylic skulls of the Amphibia. 



In the ordinary Plagiostomes, on the other hand, the palato- 

 quadrate becomes smaller and more freely united with the skull. The 

 otic process ( = spiracular cartilage) is smaller, and the hyomandi- 

 bular takes a larger and larger share in suspending the mandibular 

 arch to the skull, which is therefore eminently hyostylic. 



Turning now to osseous fishes, the skull which presents the nearest 

 resemblance to Ceratodus is that of Polypterus. This will be obvi- 

 ous to any one who studies the excellent description of the chondro- 

 cranium of this fish given by Dr. Traquairf. 



But in the character of the palato-quadrate arch and the great 

 relative size of the hyomandibular, the skull is as thoroughly and 

 completely hyostylic as is that of ■ any other existing Ganoid or 

 Teleostean. 



Thus, having regard only to the structure of the cranium, the 

 relations of Ceratodus may expressed as follows : — 



Amjihibia. ____— — -Ganoidei. Teleostei. 



Ceratodus.: 

 Chimcera. 



•Cestracion. Raia. 



Notidanus. 



- v v- 



AUTOSTYLICA. AMPHISTYLICA. HyOSTYLICA. 



* P. Z. S. 1874, pi. xxxi. figs. 1 & 2. 



t " The Cranial Osteology of Polypterus" Journal of Anatomy and Physi- 

 ology, 1870. 



