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



sequence to bear on that in a different sequence, except by the inter- 

 position of a genealogical table more or less complete. And this 

 130ssibility of making one view confirm another arises, of course, from 

 the fact that, although we may consider the histories of heart, of 

 brain, of skull, or of limb, each by itself, still each heart and limb was 

 only a part of an animal in which were all the other parts. 



I therefore here introduce the following views respecting the affini- 

 ties of the groups involved in this discussion, views in which there is 

 little more original than an independent judgment. I give them 

 without the proof, which is extremely bulky for its weight. 



The Gnathostomes and Stapedifera* are geneticf groups. 



The Gnathostomes minus the Elasmobranchs are a genetic group. 

 The nasal fossa is bridged in all the members of this group, with very 

 few and insignificant exceptions.^ It is never bridged in the Elasmo- 

 branchs. We will call the members of this group then Gephyrrhina 

 [Fecpvpa =z a bridge). 



* Stapedifera=Amniota + Amphibia. In my paper of last February I referred inci- 

 dentally to the stapes as formed from the proximal part of the hyoid arch. This was 

 a mistake, and my attention was called to it by the following passage from Wieders- 



];;ieim: — " so ist es beinahe traditionell geworden, die bei der nngleich besser 



studirten Entwickelungsgeschichte der Anuren iiber den Schallzuleitungsapparat 

 gewonnenen Resultate ohne Weiteres auch auf die Urodelen auszudenen. Darauf 

 beruht die in die verschiedensten Abhandlungen und Lehrbiicher iibergegangene 

 Behauptung: 'das Operculum der Urodelen hat sieh vom Hyoidbogen abgeschniirt.' " 

 (R. Wiedersheim, Kopfskelet der Urodelen, Morph. Jahrb. Band, III, 1877.) Wieders- 

 heim proceeds to show that the stapes or " operculum " of the Urodela is cut out of 

 the cartilage of the periotic capsule as a disk tilling the fenestra ovalis, from which a 

 boss, more or less prolonged into a rod in the various species, arises by continuous 

 spreading of the cartilage. He refers to Parker as having arrived at the same results. 

 But it appears that this is only half of the story. Those invaluable memoirs of 

 Parker on the skull, for which every living anatomist must bear a feeling of personal 

 gratitude, seem to establish beyond all reasonable doubt that the stapes of mammals 

 and what corresponds to it in the Stapedifera amammalia is never developed from the 

 hyoid arch, but always from the periotic capsule. Moreover, it appears that in the 

 Batrachia, or tailless Amphibia, and in the Amniota amammalia this periotic stapes 

 connects itself with the proximal part of the hyoid arch to form the columella ; and it 

 seems to be proved that this proximal part of the hyoid arch may sometimes originate, 

 ontogenetically, neither in continuity with the distal portion nor at the same time with it 



f By a genetic group I mean an assemblage of animals, or plants, which are more 

 nearly related to one another than any of them is to anything outside the group. Thus 

 the latest common ancestors of any two members of any genetic group would be later 

 than the latest common ancestors of any form in the group and any form outside. 



± The Chromides, except Symphysodon, and the Labroidei ctenoidei or Pomacentridaj, 

 have only one external opening on each side. There are also some curious modifica- 

 tions of the olfactory apparatus among the Tetrodontina. 



