A CTINOP TER YGII 307 



sinus opening behind. In the Teleostei, however, the sinus may 

 disappear, all the openings coming to the surface (Fig. 354). 



The Actinopterygii first appear in early Devonian strata, and 

 have since become the dominant group of fish. The recent 

 advances in our knowledge of the structure and classification of 

 the more primitive fossils formerly included in the Ganoidii are 

 due to Wagner, Vetter, Thiolliere, Pictet, Heckel, Zittel, Pander 

 [312], Traquair [446, 448, etc.], and A. S. Woodward [505]. They 

 are now generally subdivided into four sub-orders of equal rank ; 

 but the Lepidosteidae and Amiidae are so closely related to the 

 Teleostei, that we prefer to unite these three groups in one Sub- 

 division, the Holostei, to be distinguished from the Chondrostei 

 (Heckel [205a], Pictet, Wagner [477], Vetter [473], Davis [100], 

 Thiolliere [473], Zittel [513], Crook [94], Kner and Steindachner 

 [268], Loomis [285]). 



Subdivision 1. 

 Order CHONDROSTEI. 



This group contains the most ancient and the most primitive 

 known Actinopterygii. The definite association of the Palaeoniscidae 

 and Platysomidae with the Acipenseroidei is due to Traquair [446, 

 448] (one of the Palaeoniscids, Gheirolepis, is found as the first 

 representative in Lower and Upper Devonian rocks). They seem 

 to have diverged in three principal directions, leading to the 

 Platysomidae, Chondrosteidae, and Catopterydae. The affinities 

 of the Belonorlrynchidae are very doubtful, and they are only 

 provisionally placed here. The degenerate Chondrostei are con- 

 sidered to be the only direct survivors at the present day of the 

 sub-order ; but it must be remembered that the belief in their close 

 affinity to the Palaeoniscids rests on slender evidence. The whole 

 endoskeleton is very incompletely ossified, remaining for the most 

 part cartilaginous, and in consequence is very incompletely known 

 in the extinct species. 



The notochord in living forms is quite unconstricted, and 

 surrounded by a very thick sheath outside which are no true 

 centra. The vertebral column was presumably of the same 

 structure in the fossils ; but traces of centra have been described 

 in Pygopterus and Pluinerostcon (Traquair [446], Fritsch [139]). Only 

 pleural ribs occur in the living Chondrosteidae ; but they are 

 unknown in the extinct families. The dorsal ribs are absent. 



In Acipenser the persistent notochord, surrounded by its thick 

 fibrous sheath and its thin elastica externa, bears along its dorsal 

 surface a cartilaginous neural tube formed by a series of large 



