474 



PEOr. ST. GEORGE MIVAET 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 LXXIX. 

 fig. 6). 



Professor Gegenbaur at first ^ derived the Elasmobranch limb from that of the Lepi- 

 dosiren type of limb as below : — 



Fis- 4. 



IP 



A. Lepidosiren type ; B. Ventral type ; C, D, E. Ray pectoral type ; F. Shark pectoral type. 



Subsequently he adopted * the Ceratodus-limh 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-. 



' L. c. p. 304. 



' See MiiUer's Axchiv, 1845, p. 3, " Die einzige Analogie dazu liefem die abgesonderten Riickenflossen des 

 Polyptems hichir, welche aua einer Flossenstange und einer davon ausgehenden Flossenfahne bestehen." 

 ' Jenaische Zeitschrift, vol. v. Heft 4, 1870. < Jenaische Zeitschrift, vol. vii. Heft 2. 



