1878.J 



Osteology of Polyodon folium. 



455 



If tested by the structure of their upper jaws, the Crossopterygidse, 

 Lepidosteidse, and the Palseoniscidae should be referred to the 

 Teleosteoidei, while the Acanthodidae would probably belong to the 

 other sub-order. Nevertheless, the distinctness of the two groups is 

 materially lessened by the existence of many annectent fossil forms. 

 Acvpenser and Ghondrosteus in the one division and Palasoniscus 

 in the other, partially bridge over the gap which exists between the 

 two when the recent forms only are considered. The Placodermi 

 and the Cephalaspidae must still be referred to as being "incertae 

 sedis." 



A comparison of the skull of Polyodon with the Amphibian skull 

 leads to interesting results. Perhaps the most remarkable feature in 

 which Polijodon resembles the Anura, is in the possession of a for- 

 wardly directed " orbitar process " associated with a suspensorium so 

 much inclined backwards that the gape is extended even beyond the 

 posterior limits of the skull. 



The condition of the "orbitar process," as an apparently function- 

 less rudiment in Polijodon, does not throw any light on its primitive 

 origin, but its position and relations in the adult Lamprey, and its 

 transitory condition in the embryo Anura, suggest that originally it 

 may have acted as an anterior suspensor to the much inclined 

 mandibular pier of animals possessing a suctorial mouth, prior 

 to the adaptation of the pterygo- quadrate arcade to that purpose. 

 These facts, together with the rotation of the quadrate cartilage, which 

 we may infer to have taken place from the direction and relations of 

 the "orbitar process," are indications of the existence of a close 

 parallelism between the developmental history of the cranium in the 

 embryo Polyodon, and in such otherwise dissimilar Anurous Amphibia 

 as Dactylethra capensis, Bufo omatus, and TLana temporaria. The 

 fenestration of the roof of the periotic capsule which exists in 

 Polyodon seems to correspond to the primitive auditory involution 

 which persists in Siren lacertina, and, as in the latter, it is situated to 

 the outer side of the arch of the posterior vertical semicircular canal, 

 and not to its inner or mesial side as is the case with the Selachians. 



The co-existence in Polyodon of so remarkable a combination of 

 Amphibian and Selachian features suggests an enquiry into the 

 phylogenetic relationship of the Ganoids, the Amphibia, and the 

 Elasmobranchs, or, in other words, suggests the question, is the 

 ancestral stem of the G-anoidei more closely related to that of the 

 Amphibia, or to that of the Elasmobranchs ? An analysis of the 

 structural features common to any two of these primary groups seems 

 to affirm the monophyletic origin of the two first-mentioned. On 

 this assumption the relation of the three groups may be roughly and 

 tentatively expressed as follows : — 



It seems not improbable that a primitive ancestral stock (x) very 



