372 PEOF. T. W. BRIDGE OIN" THE MORPHOLOGY OF THE SKULL 



mandibular forms the common suspensorium of the mandibular and hyoid arches, 

 and is associated with a hyostylic condition of the skull. The small size of the 

 hyomandibular element in Ceratodus may be due to its reduction to the condition of a 

 purely vestigial structure, but none the less does the noteworthy fact of its retention in 

 the most primitive of existing Dipnoi suggest that it may be regarded as a remnant 

 of the primitive hyostylic condition of the skull in the ancestors of the Dipnoi i. A 

 similar significance may also be attached to the hyomandibular of Chimmra, at all 

 events to the extent of suggesting the origin of the Holocephala from hyostylic 

 ancestors. For these reasons I am inclined to agree with Dollo that autostylism is a 

 purely adaptive modification, and may occur independently in diverse groups of Fishes 

 wherever any advantage is to be gained from the fixation by fusion to the skull of the 

 primitive elements of the upper jaw (palato-quadrate cartilage) for the purpose of 

 providing the needful support for a massive and peculiar dentition, or even, as I have 

 suggested above, for a system of labial cartilages in a suctorial mouth. 



While agreeing with Dollo as to the adaptive character of autostylism, I may add 

 that this does not necessarily imply agreement with his views as to the phylogenetic 

 origin of the Dipnoi from the Crossopterygii. The theory is eminently controversial, 

 and, although the author has much to say in its favour, there are nevertheless, in 

 my opinion, weighty objections to its implicit acceptance, as well as no less cogent 

 reasons for assigning to the Dipnoi a more remote ancestry. My object in referring 

 to this question is rather to point out that autostylism in itself must not necessarily 

 be regarded as an indication of genetic affinity. The necessity for caution in this 

 respect is illustrated by certain recent views as to the supposed affinity of the 

 Arthrodira (Coccosteus, DinicJithi/s, and their allies) to the Dipnoi. Thus, Smith 

 Woodward [45], in discussing the affinities of the latter group, remarks : " the evidence 

 in favour of the autostylic character of the Coccostean Fishes has now accumulated to 

 such an extent that we venture to regard them as an order of Dipnoi " [l. c. p. xxi). 

 It is quite possible that further research may confirm the accuracy of these views, but 

 for the present I venture to think that their ultimate proof must rest on other grounds 

 than the occurrence of autostylism. 



' "With reference to this point the following remarks by Gadow [9 a, p. 459] may he quoted : " It is highly 

 prohable that in the ancestral Dipnoi the hyomandihula was much larger, and that it was already, as in 

 Teleosteans, broken up into a proximal cranial persisting part, and into a distal or symplectic element, which 

 later on, when the hyostylic support of the jaws was superseded, either persisted IProUus, &c.], or tiltimately 

 became lost [Ceratodus, Salamandrina]." 



