242 J. K. Thacher — Ventral Fins of Ganoids. 



diverging from O seems a fit way of symbolizing the fact tliat the 

 point of union of C witli the other lines came so near O that in our 

 present ignorance we can not distinguish it from O. And this, I take 

 it, has been the practice hitherto. 



From this examination of the Ganoid ventrals liave been obtained 

 the following results, for the most part merely confirmatory of views 

 previously expressed. 



I. The hind limbs with their arches have been derived from a series 

 of simple rays divided into three segments. 



II. The " Archipterygium " never existed outside of the Choanate 

 line, and probably never outside the Dipnoan. 



III. While the two former results are well established and may be 

 regarded as final, the views which we must form respecting the \n\n\- 

 ber of rays, the amount of concrescence between adjacent rays, the 

 union of the two pubes and the development of the iliac process are 

 much more indefinite and uncertain. It seems probable that there 

 was a reduction in the number of the rays, in going from the Proto- 

 gnathostomi to the Protogephyrrhina, and perhaps a still farther 

 reduction in the line leading off toward the Teleosts. It is not un- 

 likely that the concrescence in each of the ancestral groups just men- 

 tioned lay between that oi /Scaphirhynchus and Polyodon. The union 

 of the two pubes is found in all Elasmobranchiates, except the Holo- 

 cephali, in some Teleosts, and in the Dipnoi. Whether this slight 

 development was independent in the different groups where it occurred 

 or not seems at present impossible to say. An iliac process is found 

 in Chimsera, in the Kays, in the Chondrostei, apparently in the 

 Dipnoi, and in the Stapedifera ; there seems to be the same doubt 

 about the history of this. But it is not very important. The essen- 

 tial fact seems to be this — that the line from the Protognathostomi 

 to the Protostapedifera was in such a state that no very extraordi- 

 nary set of circumstances was sufficient to evoke an iliac process. 



Index to Plates. 



[Figures all drawn with a camera.] 



Plate I. 



Fig. 1. Ventral of Acipe'i^er brevirostris, from dorsad. 



Fig. 2. Partially ideal figure of the same, obtained by combining previous figure with 



figure 64, PI. LIX, Trans. Conn. Acad., vol. iii. 

 Fig. .3. Ventral of Polyodon folium, from ventrad. A. Single ray from middle of fin 

 showing iliac process, a'. 



Plate II. 

 Fig. 4. Ventral of Polypterm hichir. 

 Fig. 5. Ventral of Lepidosteus osseus, from dorsad. 

 Fig. 6. Ventral of Arnia calva, from dorsad. 

 Fig. 7. Ventral of Scaphirhynchus cataphractus, from dorsad. 



