474 PROF. ST. GEORGE MIVART ON THE 
mobility,” has resulted from “ the lengthening of the axial skeleton, accompanied by a 
removal of its distal elements further away from the shoulder-girdle, and a diminution 
in the number of its rays.” At the same time the possibility should be borne in mind— 
a possibility which, I am inclined to think, should rather be deemed a probability—that 
such forms as Ceratodus and Lepidosiren may have arisen as a special lateral offshoot 
from the main stem, an offshoot not leading to the parents of Batrachians and higher 
Vertebrates. Mr. Thacher suggests! the possibility that “the fringing rays are new 
developments,” and is “strongly inclined to suspect that the three portions of the 
second piece of the limb of Ceratodus, which Giinther describes, indicate three fin-rays, 
and that the feathering of one of them is a later development.” Dr. Peters long ago” 
pointed out (a fact adverted to by Mr. Thacher) that there is a resemblance between 
the fringing rays of the Dipnoi and the secondary filaments borne by the dorsal-fin 
rays of Polypterus, which are unquestionably new developments (Plate LX XIX. 
fig. 6). 
Professor Gegenbaur at first? derived the Elasmobranch limb from that of the Lepi- 
dosiren type of limb as below :— 
7. 
IN 
| 
f. 
I) 
7) 
T 
“fy 
LLL) 
“A 
Y 
7] / 
Y) 
yf 
/ 
LL 
aS 
—— wy, 
Y 
4 

——s 
= 
A. Lepidosiren type; B. Ventral type; C, D, E. Ray pectoral type; F. Shark pectoral type. 
Subsequently he adopted* the Ceratodus-limb as the fundamental form, explaining 
the formation of the Elasmobranch pectoral as due to the great increase in length and 
coalescence of the diverging radials of one side, these growing into the pro-, meso-, 
» Lie. p. 304. 
* See Miiller’s Archiv, 1845, p. 3, “‘ Die einzige Analogie dazu liefern die abgesonderten Riickenflossen des 
Polypterus bichir, welche aus einer Flossenstange und einer dayon ausgehenden Flossenfahne bestehen.” 
3 Jenaische Zeitschrift, vol. vy. Heft 4, 1870. 4 Jenaische Zeitschrift, vol. vii. Heft 2. 
