SKELETON OP LEPIDOSTEUS. 457 



ways. In the EiignathidaB the cleft of the mouth extended back- 

 wards and the suspensorium became vei^tical or directed back- 

 wards; in these strong-swimming fishes the dorsal and anal rays 

 have increased in number and become concentrated, and the 

 caudal has become forked. On the other hand, in the Lepido- 

 steidte the snout and lower jaw have grown forward, the quadrate 

 retaining its anterior position, and we may suppose that as the 

 lower jaw became longer and more powerful the strain on the 

 suspensorium was relieved by the acquirement of the articulation 

 of the metapterygoid with the parasphenoid. Other modifica- 

 tions, such as the great development of the interoperculum, seem 

 to be related to the elongation of the orbital region, which gives 

 an increased surface for attachment of the strong mandibular 

 muscles. In these sluggish, lurking fishes the fins remain essen- 

 tially as in the Semionotidse. 



The Aspidorhynchidse, which have sometimes been associated 

 with the Lepidosteidae, have none of the peculiarities of that 

 family. An analysis of their charactei-s shows a close agreement 

 with the Eugnathidae, from which they difier in the prolonga- 

 tion of the prajmaxillaries to form a beak and the development 

 of a prsesymphysial bone in the lower jaw. Especially important 

 is the resemblance of the symplectic to that of the Eugnathidje 

 and Amia (Smith Woodward, Monogr. Palaeont. Soc. 191u-1919, 

 p. 98, pi. XX. fig. 2). 



It is well known that the Teleosteans agree with the Holostean 

 Ganoids in the structure of the dorsal and anal fins, the absence of 

 clavicles (infra-clavicles), and the structure of the suspensorium 

 and opercles. But the most primitive Teleosteans (e. g. the 

 Elopidse) differ from the Holostei in several important respects, 

 as follows: — Tlie caudal fin is homocercal*. There ai-e three 

 coracoid bones (hyper-, hypo-, and meso-coracoid), wliereas in 

 Lepidosteus and Amia there is not more than one (hypercoracoid 

 or " scapula ") and the mesocoracoid bridge is cartilaginous. The 

 pectoral radials are all inserted on the hypercoracoid and hypo- 

 coracoid, the lowest radial probably representing the metaptery- 

 gium. An endochondral supraoccipital bone is present. The 

 vomer is unpaired. The lower jaw has no "supra-angular" aiid 

 no intra-dentaries, and the preearticular is reduced t. The prse- 

 maxillaries are loosely attached, i-emote from the frontals, and in 

 front of the olfactory sacs. 



* Regan, Ann. Mag. Nat. Hist. (8) v. 1910, p. 353. 



f The cun-ent nomenclature of the Teleostean lower jaw is faulty. The bone 

 usually called angular is the homologue of the lower articular of Amia and the single 

 articular of Polypterus. Ridewood's " endosteal articular " is the homologue of the 

 upper articular that articulates with the quadrate in Lepidosteus and Amia. His 

 " ectosteal articular " is the angular, and his " sesamoid articular " in all pro- 

 bability the prseartioular. 



According to Ridewood (P. Z. S. 1904) the bone last named serves for the attach- 

 ment of the tendon of part of the levator muscle, but it may well have persisted 

 rather than have been developed for this purpose. In the Cyprinodonts the opisthotic 

 is reduced to a nodule to which the lower fork of the post-temporal is attached. 



