DR. A. GUNTHER’S DESCRIPTION OF CERATODUS. 
538 
destined to serve as a framework to the surrounding soft parts, and as a support to the 
rays of the fold surrounding the fin. 
I may add here that the muscle moving this fin imitates the form of the cartilaginous 
framework ; a flat, broad band covers and runs along the central axis, sending off towards 
the right and left small fascicles, one to each cartilaginous branch. 
When I designated the arrangement of the parts of this pectoral skeleton unique, I 
did not mean to convey the idea that no homological relation could be pointed out be- 
tween the parts of the pectoral skeleton of Ceratodus and that of other fishes. It is 
quite evident that we have here a further development of the simple pectoral axis of 
Lepidosiren in the direction towards the Plagiostomes. The pectoral skeleton of Lepi- 
dosiren paradoxa consists merely of the central series of cartilages of Ceratodus ; there 
is no fin-like expansion of the skin of the pectoral limb, which is a simple tapering 
filament. In Lepidosiren anneetens this pectoral filament is bordered by an expan- 
sion of the skin along the lower edge ; and even minute fin-rays are imbedded in each 
lamina of the fold ; in order to support this low one-sided rayed fringe, very small, 
single-jointed cartilages are added to the axis*. The fin is still more developed in 
Ceratodus : it has become a broad, scythe-shaped paddle, dilated by a fold of the skin, 
with two layers of fin-rays surrounding it in its entire circumference ; therefore sup- 
porting cartilaginous branches are added on both sides of the axis ; and most of the 
branches are composed of several joints, in order to reach the more distant parts which 
require the support. 
The arrangement of the limb-skeleton of Ceratodus is foreshadowed in the pectoral fin 
of Acipenser. In the left fin of an Acipenser sturio, var. oxyrhynchus , four cartilaginous 
rods are attached to the basal cartilage (a ) ; the innermost ( b ) consists of three joints, and 
represents the jointed axis of the Ceratodus-limb ; it is provided with three two-jointed 
branches (c7), but on one side only, namely the outer, the branches being intercalated 
between their axial base and the next cartilaginous rod. Having compared the arrange- 
ment in Ceratodus to a diphycercal tail, we may designate the plan observed in Acipenser 
as offering analogy with heterocercy. The three outer rods (c) are two-jointed, the pos- 
terior joint being much the smaller, and obliquely bent inwards. The outermost rod 
Four or five of these ray-bearers are obliquely attached to each joint of the axis. Peters, Miill. Arch. 1845 
Taf. 2. fig. 2. 
