532 
DR. A. GUNTHER’S DESCRIPTION OF CERATODUS. 
slightly curved cartilage ( m ) ; its proximal end has a glenoid cavity, fitting into the 
humeral condyle ; the joint is simple, free, allowing of a considerable amount of motion, 
its parts being held together by a ligament fastened round its circumference. This is 
the only true joint in the limb, all the other parts being fixed to one another by con- 
nective tissue. I consider this cartilage to be the forearm ; a horizontal section along 
its longitudinal axis does not show any primary division. The next following cartilage 
(marked a in the accompanying woodcut) forms the base of the paddle ; although exter- 
nally it appears as a single flat, broad, short piece, unevennesses of its surface indicate 
that several primary pieces are coalesced in it. I am confirmed in this view by a hori- 
zontal section, in which the lines of the former divisions are preserved in the shape of 
tracts of a white connective tissue. Three such divisions may be distinguished, corre- 
sponding to the three carpals of most Plagiostomes*. If this determination is correct, 
then the antibrachial cartilage just described is not represented in that order. 
The remaining framework of the paddle shows an arrangement unique among the 
Vertebrata. From the middle of the basal cartilage a series of about twenty-six sub- 
quadrangular pieces takes its origin, forming a longitudinal axis (b) along the middle 
of the paddle to its extremity; the pieces become gradually smaller, and are scarcely 
distinguishable towards the end of the paddle. On the two posterior corners of each 
piece a branch ( d ) is inserted, running obliquely backwards towards the margin of the 
fin; the branches of the first eight or twelve pieces are three-jointed, the remainder 
two-jointed, the last having no branch at all. Slight irregularities, such as the origin 
of two branches from one side of a central piece, occur, as also several four-jointed 
branches ( c ) being inserted immediately on the basal cartilage. As in the vertical fins, so 
also in the pectoral, the fin-rays are very fine, imbedded in the skin, and arranged in two 
layers, between which the ends of the cartilaginous branches are received. The analogy 
of this framework to that of the caudal portion of the vertebral column is striking. 
Ceratodus is not only truly diphycercal as far as the termination of the body is concerned, 
but this term may be also applied with regard to the extremity of its paired fins. The 
many-jointed pectoral axis may be compared to the series of neural and haemal apo- 
physes, both forming the base to a system of superadded processes (here two- and three- 
jointed branches, there neural and interneural, haemal and interhsemal spines), which are 
* Pro-, meso-, and metapterygium of Gegekbaur. 
