187(i.] PROF. T. H. HUXLEY ON CERATODUS FORSTERI. 41 



are all connected together by a strong ligament, which is continued 

 to the pectoral arch. Moreover a small styliform cartilage passes 

 from the last angle to the pectoral arch, and is connected with the 

 dorsal end of the fifth branchial arch. It appears to represent the 

 dorsal element of that arch. 



Johannes Miiller, fully appreciating the importance of the differ- 

 ences between the skull of the Chimseroids and those of other 

 " Elasmobranchii," and sagaciously remarking that "the skull of 

 Chimcera is most hke that of a tadpole"*, was thereby led to separate 

 the Chimaeroids as a suborder of the Elasmobranchii under the name 

 o^ Holocephali. It appears to me that he might have been justified 

 in going still further ; for, considering, in addition to the crariial 

 characters, the structure of the vertebral column and of the branchiae, 

 the presence of an opercular covering to the gills, the peculiar den- 

 tition, the almost undeveloped gastric division of the alimentary 

 canal, the opening of the rectum quite separately from and in front 

 of the urinogenital apertures, the relatively small and simple heart, 

 the Chimaeroids are far more definitely marked off from the Plagio- 

 stomes than the Teleostei are from tlie Ganoidei. 



In all other Fishes, except the Marsipobranchii, the mode of con- 

 nexion of the mandibular arch with the skull is different from that 

 which obtains in the ChiniEeroids and the Dipnoi. The palato- 

 quadrate cartilage is no longer continuous with the chondrocranium 

 (though the bony elements of that arch may unite suturally with 

 those of the skull, as in the Plectognathi), but is, at most, united 

 with it by ligament. Moreover the dorsal element of the hyoidean 

 arch, or the hyomandibular, usually attains a large size and becomes 

 the chief apparatus of suspension of the hinder end of the palato- 

 quadrate cartilage with the skull: Skulls formed upon this type, 

 which is exemphfied in perfection in Ganoidei, Teleostei, and ordi- 

 nary Plagiostomes, may therefore be termed hyostylic. 



But though the typical forms of autostylic and hyostylic skulls, 

 as exemphfied, e.g., l)y a Sturgeon, a Pike, and a Dogfish or Ray, 

 on the one hand, and' Chimtera, Ceratodus, and Menobranchus on 

 the other, are thus widely different, certain Plagiostomes present A 

 condition of the cranium which tends to connect the two by a middle 

 form, which may be termed amphistylic. 



In the amphistylic skull the palato-quadrate cartilage is quite 

 distinct from the rest of the skull ; but it is wholly, or almost wholly, 

 suspended by its own ligaments, the hyomandibular being small and 

 contributing "little to its support. The embryo amphibian is am- 

 phistylic before it becomes autostylic ; and, in view of certain palseon- 

 tological facts, it is very interesting that the link which connects the 

 amphistylic with the ordinary Selachian skull is that of Cestracion 

 (fig. 8). 



If the palato-quadrate cartilage of ChbncBra were membranous in 



the centre, as it is in the tadpole, and if along three lines radiating 



from this centre the cartilage were converted partly into fibrous tissue 



and partly into a true joint, the result would be to produce a palato- 



* ' Vergleichende Anatomie der Myxinoiden,' erster Theilj p. 150. 



