20 



PALEONTOLOGY OF OHIO. 



derms and the modern' Siluroids. Prof. Owen had previously called at- 

 tention to this resemblance,* though rather to explain the function of the 

 plate armor of the Placoderms, than to prove a zoological affinity. In fact 

 this resemblance is in many respects very striking ; so much so that it is 

 difficult to believe that it is simply " homoplastic," and not " homogenetic." 

 Assuming the theory that our modern Fishes, as well as Amphibians and 

 Keptiles, are derived from a common ichthyic stock, it is not illogical to sup- 

 pose that the characters of thatancient stock have been divided among their 

 descendants, one line taking one, and another, another of the most striking 

 features, and handing them down as heirlooms through generation after 

 generation. The plate armor of the ancient fishes doubtless served a very 

 useful purpose, and there has probably never been, in the history of fishes, 

 a " piping time of peace," when they could all " lay their armor down," 

 but some tribes have constantly worn it, either in the old time-honored 

 and well-tried patterns, or modified to suit changed circumstances. 



Thus it may very well be that the plate armor of the old Placoderms 

 has become the heritage of the Siluroids, and they have retained little 

 beside ; while the dentition and much of the internal structure have been 

 preserved by the far more old-fashioned fishes, the Di])noi. 



In Professor Huxley's memoir " On the Classification of Devonian 

 Fishes," outline figures are given of the pectoral shields of Coccosteus, 

 Clwrias, and Lorica/ria, which are reproduced below. 



-^ c^y^ 



OUlBIAS. Coocostedb. Loeioabia. 



a a a, Hyoid bones, b V 6, Urohyals. c c' c, •• Coraooias." d d' d, " Eaflii." 



From the text which accompanies these figui'es the following passages 

 are quoted : 



" On comparing this apparatus with the sternal shield of Coccosteus, one 

 is tempted to compare the antero-median piece of the latter with the 

 urohyal of the Siluroid, the antero-lateral piece \nth the ' cor'acoid,' and 



* Palaeontology, Second Edition, p. 148. 



