1876.J PROF. T. H. HUXLEY ON CERATODUS FORSTERI. 43 



branchia, Cestracion exhibits a lower stage of organization than 

 Chimcera, in which, as in Ceratodus, the mandibulo-hyoid cleft has 

 disappeared. On the other hand, the hyoidean arch presents a form 

 intermediate between that of the ordinary Selachians and that of 

 Ceratodus and ChimcBra. It is stout ; and its dorsal element, still 

 retaining a httle of its original form, but much thicker and more cy- 

 lindrical, is no longer united with the skull by hgament and muscle 

 merely, but articulates with a process of the underside of the periotic 

 capsule. Moreover its distal end is connected by strong ligamentous 

 fibres with the posterior end of the palato-quadrate cartilage and with 

 an inward process of the articular end of the mandible (the sustenta- 

 culum of Gegenbaur). 



In fact, the " epibranchial " of the hyoidean arch of Cestracion is 

 just beginning to take on a new function, that of suspending the 

 palato-quadrate cartilage and mandible to the skull. It is a true 

 hyomandibular, though small and insignificant relatively to what it 

 becomes in other Plagiostomes, in Ganoids, and in Teleostei. 



Had 1 been acquainted with the skull oi Cestracion in 1858, I 

 should have been spared the hesitation which I then felt* as to iden- 

 tifying the hyomandibular of Fishes with the summit of the hyoidean 

 arch, and which has subsequently been removed by abundant evi- 

 dence published by Mr. Parker and myself. • 



In the general form of the skull, the position and proportions of 

 the olfactory capsules, and the characters of the principal labial and 

 alinasal cartilages, Cestracion has a stronger resemblance to Chimcera 

 than is exhibited by any other Plagiostomes ; and I take it to be one 

 of the lowest of Selachian skulls. 



I am aware that in expressing this opinion I am diametrically op- 

 posed to Gegenbaur t, whose elaborate study of the Plagiostomeskull 

 entitles his opinion to the greatest weight, and who regards Cestra- 

 cion as possessed of one of the highest of skulls in its group, while 

 Heptanchus and Hexanchus have the lowest. 



There is a certain ambiguity about the terms " highest " and 

 " lowest ;" but if by the former we understand the most extreme 

 modification of the hyostylic type of skull characteristic of the group, 

 then I should have no hesitation in regarding the skulls of the Rays 

 as the highest of Plagiostome skulls, while Cestracion represents a 

 low form of the autostylic type. 



Notidanus, on the other hand, appears to me to have an essentially 

 low form of skull, so far as it is more completely amphistylic than 

 any ordinary Plagiostome ; but on this low form is superinduced a 

 modification by which it approaches the higher autostylic skull. 

 This is the union of the palato-quadrate arch with the postorbital 



* Croonian Lecture, 1858, and Lectures on the Theory of the Skull, 1864. 

 See also " On the Malleus and Incus," P. Z. S. 1869 ; ' Manual of Vertebrate Ana- 

 tomy,' 1871, p. 85; and Mr. Parker's "Memoir on the Development of the 

 Salmon," Phil. Trans. 1872. 



t ' Das Kopfskelet der Selachier,' p. 60. In controverting the opinion of 

 Professor Ovf en that the Cestracion is less advanced in cranial development than 

 Squatina, Gegenbaur observes, " So mochte ich gerade das Gegentheil behaupten, 

 und nicht etwan bloss beziiglich der Basalverhaltnisse des Craniums." 



