54 IOWA GEOLOGICAL SURVEY 



Like Eurypterids and certain other contemporary inverte- 

 brates accompanying them in the same fauna, the earliest recog- 

 nizable fish-like vertebrates, that is to say, Silurian Ostra- 

 cophores, appear to have been mere mud-grovellers inhabiting 

 the bottom of shallow seas. Imperfectly equipped for loco- 

 motion, and making little progress throughout their subsequent 

 history in the direction of improved swimming-organs, their 

 evolutionary advance took place along lines immediately condi- 

 tioned by their sedentary mode of existence. Thus, their 

 growth-energy seems to have been expended chiefly in the elab- 

 oration of a protective exoskeleton, constituted in the more 

 primitive forms of dermal granules or tubercles scattered 

 throughout the tough but flexible integument, but becoming fused 

 and segregated by successive stages into hard, sometimes even 

 heavy plates, arranged according to a definite pattern. The cul- 

 mination of their progress fell short, however, of the develop- 

 ment of calcified endoskeletal structures, of paired limbs homol- 

 ogous with those of all higher vertebrates, and of completely 

 formed jaws articulating with the cranium and functioning in the 

 normal manner. These lowly fish-like organisms are, therefore, 

 chiefly interesting in that they inform us regarding the manner 

 in which a hard skeleton was first acquired among vertebrates, 

 and illustrate successive stages of its elaboration. 



Fortunately very satisfactory records of this evolutionary 

 history are now available, a number of well preserved examples 

 of primitive Ostracophores having recently become known from 

 the Upper Silurian of Scotland,* and Lower Devonian (Huns- 

 riick slates) of Rhenish Prussia. t Our knowledge of the 

 skeletal modifications displayed by the lower types of Ostra- 

 cophores is due chiefly to the researches of Dr. Ramsey H. Tra- 

 quair, dean of Scottish pakeichthyology, and to the brilliant gen- 

 eralizations based upon them that we owe to Dr. A. Smith Wood- 

 ward, than whom is no higher authority or more experienced 

 student of fossil fishes. Valuable enlightenment has also been 



*Traquair, R. H., Report on Fossil Fishes collected by the Geological Survey 

 of Scotland, etc. Trans. Royal Soc. Edinburgh, 1S99, 39, pp. 827-864. — Supplemental 

 Report, Ibid., 1905, 40, pp. 879-888. 



tTraquair, R. H., The Lower Devonian Fishes of Gemiinden. Trans. Rov. 

 Soc. Edinburgh, 1903, 40, pp. 723-739.— Supplement, Ibid., 1905, 41, pp. 469-475. 

 Also short papers in Geol. Mag. for 1900 and 1902. 



