294 Evolution and Distribution of Fishes 



fully in Memoirs of the geological surveys for New York 

 {^9-93) and for Iowa {igS: 158) respectively. These are 

 a continuation of two still earlier papers (222: 131 ; 225:- 

 I ) . He there inclines to divide the group into three main 

 lines of descent. But so far as the writer can judge, he 

 would incline to accept two only, a scaleless or arthrodiran, 

 and a cycloid-scaled or dipnoan group division. 



As to first origin of the groups, the view that they 

 originated in the late Silurian period, as a type that com- 

 bined characters of Birkenia and of related primitive fishes, 

 with those of some primitive members of the Crossop- 

 terygii has much in its favor. If the calcified plates 

 described by Barrande as Coccosteiis and Homosteiis are 

 correctly diagnosed then the group has left remains in 

 the Silurian of Northern Central Europe. But in Lower 

 Old Red strata it had already split up into two well-marked 

 divisions the scaleless or arthrodiran and the scaled 

 or dipnoan lines. As representatives of the former all 

 became extinct, so far as we know, during Old Red times, 

 they may first claim attention. 



In freshwater lakes of the Russian, German, and Scotch 

 areas, abundant — at times teeming — remains of Coccosteus 

 (Fig. 9d. p. 119), Brachydirus, Phlyctaenaspis, and Chelyo- 

 phones lived, and now cover with their remains the strata 

 then deposited. This is specially true for Coccosteus minor 

 of the Caithness rocks. This species varied from 3 to 18 

 inches in length; and in the absence of scales, in the rudi- 

 mentary pectoral fins, in the persistent notochord and in 

 other characters, a primitive condition is indicated. But 

 the complex nature of the cranial, and the post-cranial 

 plates, suggests cumbrous defensive specialization as in the 

 "ostracoderms" of the same age already studied. Exten- 

 sion of Coccosteus and of Phlyctaenaspis westward into 

 the Canadian and eastern United States regions, seems 

 later to have been effected, so that the former has been 

 described by Newberry from the freshwater Sandusky or 

 Delaware strata, that are of Mid-Devonian age. But 

 when he says {224: 34) "if sought for in the shore and off- 

 shore deposits of the Chemung, Catskill, and Vespertine 

 of Pennsylvania and New York, the remains of Coccosteus 



