EXTINCT FAMILIES. 



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the dermal tin-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 Gyroptychius, 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, ami 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 has not yet 

 been discovered. 



