The Chondrostei and Holostei 317 



It may be alike indicative and suggestive to observe 

 here that there are four existing genera. Acipenser, the 

 group of the sturgeons, includes about 16 species over the 

 northern hemisphere. Some of these are freshwater, but 

 most are now anadromous in habit, and so pass out into 

 the sea to feed, but return to interior river areas to spawn. 

 Scapliirhynchiis, the genus of shovel-nosed sturgeons, in- 

 cludes four species, one of which lives in south-western 

 rivers of North America, while the other three that by 

 some authors are made into a separate genus (Kessleria) 

 occur in rivers of Central Asia. Polyodon, the spoonbill, 

 occupies nearly the same area as the American Scaphirhyn- 

 chiis, and PsepJiynis is found in the rivers of China. There 

 is still much to be learned as to the last three. 



The oldest known representative of the Chondrostei 

 is Cheirolepis trailli. This is one of the fishes noted by Flett 

 (22) and Goodchild (77:591) as typical of the Acha- 

 narras-Cromarty-Stromness beds. These, as already stated 

 (p. 120), were all of freshwater habitat. This equally ap- 

 plies to the only other and more recent species C. canaden- 

 sis from the upper Devonian of Scaumenac Bay in Canada. 

 So far then as present facts go, the Chondrosteans are first 

 known in freshwater areas. 



The genera Canobius, Phanerosteon, Holurus^ Nema- 

 toptychiiis, Rhadimchthys, Elonichthys, Cycloptychius, 

 Acrolepis, and Gonatodiis all appear first in strata of 

 Calciferous Sandstone age, and the first four disappear with 

 its close. The others persist however, up into Carbonif- 

 erous Limestone age {Gonatodiis) , or even into the Coal 

 Measures. They are all absent however from the marine 

 lists given by Davis (205: 614; 207), Traquair (77:690), 

 and others, but are continuously noted from freshwater 

 beds. Thus in Traquair's lists while he frequently speaks 

 of "estuarine fishes" the assemblages given by him from 

 Wardie Beach, from the freshwater of Burdiehouse, also 

 from the Straiton and Pentland oil shales, are clearly all 

 lacustrine — not estuarine — groups. For the presence along- 

 side them of Dipnoans, of Crossopterygians, of other 

 Chondrosteans yet to be studied, as well as of a varied land 

 flora, is thoroughly indicative. 



