528 



MR. J. S. BUDGETT ON THE STEUCTUKE 



The ShouUer-fjirdle and Pectoral Fins.—Thc cartilaginous sliouldcr-girdle consists 

 of a compact mass of cartilage of considerable thickness anteriorly, and, though thinning 

 out ventrally, of no great extent in a dorso-ventral direction (PI. XXXIII. fig. 7). 

 Posteriorly, it is prolonged as a thin plate of cartilage, towards the hinder border of 

 which is the great coracoidal foramen (PL XXXIII. figs. 7 & 8, Corf.). Above this 

 foramen is situated the protuberance bearing the convex facet which articulates with 

 the proximal ends of the pro- and metapterygial cartilages of the pectoral fin. From 

 behind, this protuberance appears as a distinct short rod of cartilage attached only^at 

 its proximal end to the flattened portion of the shoulder-girdle (PI. XXXIII. fig. 7). 

 In reality, however, it is continued forwards into the anterior thick portion of the 

 girdle as a thickened ridge. 



1. 



Text-fig. 3. 

 2. 



Meta 



P0LYPTERU6 



"~ HEPTANCHUS 



LARVAL POLYPTERUS 



Skeleton of pectoral fin of Polypterus and Heptanchiis. 



Pectoral fin of — 1. Adult Polypterus. 



2. Larval „ 



3. Heptanchus. 



Meta. Metapterygium. Mes. Mesopterygium. Pro. Propterygium. 



The pectoral fin of Polypterus is one of its most characteristic features, being unlike 

 that of all other Vertebrates excepting its congener Calamoichthys. It is therefore 

 a question of considerable interest as to what is the relation of this fin to other known 

 types, and how has it been formed. Upon this question two views have been held : — 

 (1) The fin of Polypterus has been derived from a biserial archipterygium of the type 

 of Ceratodus by the shortening of the axis and the shifting of the proximal radials up 

 the axis to form the meta- and propterygia. This view has been mainly held by 



