18 BULLETIN : MUSEUM OF COMPAKATIVE ZOOLOGY. 



oped. It is at least where traces of them should be looked for amongst 

 the most primitive genera. 



Autostyly. — No Arthrodire thus far discovered can be definitely 

 proved to have been autostylic. Tliis type of cranial structure is to be 

 inferred, however, (1) from concurrent testimony of other characters 

 pointing to Dipnoan relationships ; (2) from the remarkable similarity 

 of the jaw-parts to those of modei'n Dipneusti ; (3) from the absence of 

 any ossification which can be interpreted as hyomandibular, even in the 

 most exquisitely preserved skeletons ; (4) from the occurrence of articular 

 cartilage in natural association with the lower jaw of Dinomylostoma ; 

 and (5) from the position and appearance of a pair of well-marked fossae 

 on the under side of the head in Macropetalichthys, described by Cope 

 as " an articular glenoid cavity, possibly for the condyle of a man- 

 dible." ^ Regarding these latter structures, it need only be remarked 

 that Cope's interpretation is materially strengthened by the resemblance 

 between the fossae and certain facets for articulation with the man- 

 dible as seen in the quadrate element of Dipterus ; ^ a resemblance 

 which serves, by the way, to emphasize the close approach made by 

 this extremely generalized form to Ctenodipterine conditions. Macro- 

 petalichthys offers in many ways a fair presentment of an ancestral, 

 synthetic type. 



We may conclude this phase of the subject by calling to mind tlie 

 caution that Dollo and Bridge have urged against attributing too great 

 significance to the occurrence of autostyly amongst fishes. Reasons have 

 been given by these authors for believing that tlie nature of the sus- 

 pensorium must not necessarily be regarded as an indication of genetic 

 affinity, and that autostyly is a purely adaptive modification. Thus, it 

 is held by Bridge that the autostylic condition of the skull " 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 ele- 

 ments 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." ^ In the opinion of the writer last quoted, it is imma- 



^ Cope, E. D. On the characters of some Palaeozoic fishes. Proc. U. S. Nat. 

 Mus., 1891, 14, p. 453. 



2 Traquair, R. H. On the genera Dipterus, etc. Ann. Mag. Nat. Hist., 1878, 

 ser. 5, 2, p. 5, Plate 3, Figs. 1-4. 



3 Bridge, T. W. On the morphology of the skull in the Paraguayan Lepido- 

 siren. Trans. Zool. Soc. London, 1898, 14, p. 372. 



