402 The American Naturalist. [May, 
the dorsal and ventral mid-lines of the body are formed by the 
junction of the bifold /amine dorsales and ventrales. He con- 
sidered that the pectorals and ventrals were certain portions of 
the lower azygos fins, prevented from uniting by the interposed 
body-cavity. Maclise had previously regarded the distal por- 
tions of the limbs as corresponding with the azygos fins. In 

An 
I —Diagrammatic as cheap ne of primitive gr derivative types of lateral and 
median ia 4, primative condition, fins continuous; B, derivative condition, fins dis- 
tinctand specialized ; D, dorsa opera Be pectoral ; ch ventral: A F,anal; SF, caudal: 
orsal; FF, second dorsal fin; from Wiedershe 
1876 Balfour® described the development of the limbs of 
Elasmobranchs as “ special developments of a continuous ridge 
on each side, precisely like the ridges of epiblast which form the 
rudiments of the unpaired fins,” and concludes from this “that 
the limbs are remnants of continuous lateral fins.” The primitive 
folds persist in the vertical and caudal fins, while the paired fins 
represent portions of such folds (Fig. 1). 
As regards their intimate structure, those of the Elasmobranchs 
and Polypterus occupy in some sort a middle term between those 
of the Teleostomata and those of the Dipnoi. In order, there- 
fore, to trace out the homologies between the segments -of the 
paired fins of the Teleostomata and those of the limbs of the 
Reptilia, the parts must be traced in lineal order through the 
following classes or sub-classes: Reptilia, Batrachia, Dipnoi, 
Elasmobranchii, Teleostomata. (Figs, 2, 3.) Naturalists dis- 
* Tbid., Vol. XI, Part I., p. 132. 

