35 o The American Geologist. December, 1903. 
The absence of rays from these post-ventral fins may be 
regarded as an evidence of arrested development or of retro- 
gression and may stand with the occurrence of a similar struc- 
ture in the second dorsal or adipose fin seen in some of the 
Salmonidae. 
It may be rank heterodoxy to even hint at the existence of 
a third pair of limbs in a vertebrate animal. But on the above 
view there is nothing monstrous or incongruous about it. It 
is anomalous, judged from the existing creation, but so was the 
pineal eve when first mooted ; yet it is an accepted fact in anat- 
omy and explains some structures that were previously enig- 
matical. Considering how little we yet know of the forms of 
palaeozoic vertebrate life it is far from improbable that other 
indications of this curious organ may yet come to light ; but 
whether the solution above suggested is correct or incorrect, 
the fact is certain and awaits explanation that these Devonian 
cladodonts possessed this singular external appendage. 
In regard to the armour of the cladodonts the testimony of 
Dr. Clark's collection is decisively in favor of the view that 
they were spineless. Not a single specimen shows any indi- 
cation of any such armament. The pectorals are absolutely 
clear on this point. The ventrals are small and seemingly al- 
together soft and no signs of any hard or bony spines are 
visible on or near them. The dorsals are not known in a satis- 
factory condition, but so far as it goes their evidence is alto- 
gether negative and it is scarcely possible that any organ so 
thick and strong as an ichthyodorulite could have existed on 
them without leaving some trace of its presence on the fossil. 
There is, however, just a possibility lurking in here of the 
presence of weapons as yet concealed. It is. however, of the 
slenderest kind as would be admitted by any palaeontologist, 
who examined the fossils themselves. 
if then, all cladodonts were spineless sharks, it must fol- 
low that the wearers of Ctenacanthus and Machaeracanthus 
are still unknown. 
The following is a summary of the species of cladodonts 
that have been described from the Cleveland shale, exclusive 
of thnsr based on isolated teeth. 
Cladodus kepleri Nby. 
Length about 33 inches with the greatest breadth about 4 to 5 inches. 
He." , .<l bluntlv rounded in front. 
