454 



Mr. T. W. Bridge on 



[June 20, 



IV. " Osteology of Polyodon folium." By T. W. Bridge, B.A., 

 Scholar of Trinity College, and Demonstrator of Compara- 

 tive Anatomy in the University of Cambridge. Communi- 

 cated by Professor Newton, F.R.S. Received May 29, 

 1878. 



(Abstract.) 



On a previous occasion* the author described the cranial osteology 

 of one of the more specialized genera of existing Ganoids, and the paper 

 now laid before the Society is an attempt to add to the relatively 

 scanty literature of this department of comparative anatomy, by giving 

 in detail an account of the skeletal structures of one of the most 

 generalized members of the group. The following are among the 

 more important of the conclusions which the facts elucidated appear 

 to warrant. 



A comparison of Polyodon with its nearest living ally, the Sturgeon, 

 demonstrates the close relationship that exists between the two forms 

 as regards their skeletal anatomy. In so far as Acvpenser differs from 

 Polyodon, it approaches the Teleostean type, nor is the latter genus 

 without indications of having undergone a degree of specialization in 

 the same direction. But not the least important characteristic of 

 Polyodon is its possession of a remarkable combination of structures, 

 usually regarded as being specially distinctive of the Plagiostome 

 Elasmobranchii. The existence of such primitive characters, which 

 are also present in Acipenser, though to a less extent, suggests the 

 desirability of giving to such facts adequate expression in any scheme 

 that may be proposed for the classification of the Ganoids. After giving 

 a resume of the views held by various zoologists as to the position of 

 these genera in their order, it is suggested that J. Miiller's two sub- 

 orders (Chondrostei and Holostei) should be retained, though it seems 

 advantageous to remodel the definitions on which they were based. 

 The various families of recent Ganoidei may be arranged in two sub- 

 orders of approximately equal morphological value, which are mainly 

 differentiated by the structure of their upper jaws. 



a. G. Selaclwidei. — Pterygoid processes united in a medio- ventral 

 symphysis beneath the basis cranii; notochord persistent and un- 

 segmented ; spiracles and mandibular branchiae present. 



Gen. Polyodon, Acvpenser, ScwpMrhynehus, Chondrosteus. 



/3. 67. Teleosteoidei. — Pterygoid processes not united with each 

 other but are connected directly, or indirectly by the intervention of 

 a palatine bone, with the prefrontal region of the skull ; vertebral 

 column generally ossified ; mandibular gills abortive. 



Gen. Amia, Lepidosteus, Polyplerus. 



* " Journal of Anatomy and Physiology," rol. xi. 



