406 The American Naturaizst. [May, 
more nearly related to each other than any of them are to any 
other vertebrate groups; and that the archipterygian form is to 
be found in the unibasal pectoral and ventral fins of the Ichthyo- 
tomi, the unibasal Crossopterygia, and the Dipnoi. (Plate xiv.) 
These are substantially the conclusions arrived at by Mr. 
Thacher," who names the group composed of the Dipnoi and 
higher vertebrates the Choanata, from their possession of “ choanz,” 
or posterior nares opening behind the lips. But the Ichthy- 
otomi and the unibasal Crossopterygia were not known to him. 
According to Gegenbaur,'’ the archipterygium, or primitive 
form of limb from which all the limbs of the Gnathostomes, or 
vertebrates with lower jaws, have been developed, is to be found 
in its parent form in the limb of Ceratodus, which consists of 
an elongated, tapering, many-jointed rod, bearing upon each side 
a series of rays. In a previous paper,“ the fin of Protopterus, 
bearing on the outer side only a series of rays, had been taken as 
the type. In the second edition of his “Grundriss,” Gegenbaur 
adheres to the idea enunciated in his later article,“ and identifies 
the metapterygium of Acanthias and other Elasmobranchs, plus 
one-only out of the many rays, with the main stem of the fin of 
Ceratodus ; while the Bea gar 2 oe ae and all the 
jase 
G. 5.—Archipterygium Æ, and transitional stages to actinopterygium A, according 
to Pet at from Wiedersheim, 

other rays of the Elasmobranch fin are regarded as developments 
from the lateral rays of a Ceratodus-like fin. By a further slip- 
ping off of the rays from the main- or stem-row, Gegenbaur de- 
rives the many-rayed fins of the Teleostomata from those of the 
Elasmobranchii, which he has previously asserted to be derived 
11 Ventral Fins of Ganoids. Trans. Conn. Acad. , IV., 1877, p. 242. 
13 Anatomy of igre pA p. 
14 Jena. Zeitschrift, Bd. VII., Hft. 2, pp. igi- 141. 
15 Ibid., Bd. V., Hft 


