EXTINCT FAMILIES. 
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the dermal fin-rays, by which they are overlapped. The suborder is represented 
by three well-defined families. In the first, which is typified by the genus 
Holoptychius, the lobes of the pectoral fins are long and acute, while the teeth 
have complex infoldings of the outer layer, somewhat after the manner of those 
of the primeval salamanders, and the scales are thin and cycloidal. The second 
family, of which Rhizodus is the typical genus, differs by the lobes of the pectoral 
fins being shorter and blunter, and also by the less complicated infoldings of the 
teeth. To this family belongs Gyroptychins, from the Devonian or Old Red 
Sandstone of Scotland. While agreeing with the last in the obtusely lobate 
pectoral fins, the third family, as represented typically by Osteolepis of the 
Old Red Sandstone, is characterised by the walls of the teeth being slightly 
infolded only at their bases, and by the scales being of the true quadrangular, 
ganoid type. Remains of these fishes occur in extraordinary abundance in the 
Old Red Sandstone of Scotland; and as this deposit is of fresh-water origin, it is 
evident that they were either fluviatile or lacustrine forms. The reason why these 
and so many other ancient creatures were enveloped in coats-of-mail lias not yet 
been discovered. 
