30 



AR THR ODIRAN L UNG-FISHES 



doselachian sharks. As recently as 1887 its members were 

 associated by Traquair with Pterichthys, but the discovery 

 of jaws, specialized dentition, fin spines, and highly evolved 

 pelvic fins at once separate this group from the lowly 

 Ostracoderms. 



American Arthrodirans, described mainly by Newberry 

 and by Claypole, have proven of especial interest. They 

 occur from the Silurian to the Coal Measures. The giant 

 predatory member of this group, DinicJithys (Frontispiece, 

 and Figs. 133-137), attained a length of ten feet. Titan- 

 ichthys, less formidable in armour and dentition, may well 

 have been twenty-five feet in length. These forms occur 

 almost exclusively in the Waverly of Ohio. Their discovery 

 has here been due to the efforts of Dr. William Clark 

 of Berea, Rev. William Kepler of New London, and Mr. 

 Jay Terrell of Linton ; and most of the type specimens 

 have been preserved in the museum of Columbia College, 

 New York. 



The European member of this group is a small, fresh- 

 water {.'') form, Coccosteiis, especially abundant in the Old 

 Red Sandstone of Scotland. It has thus far yielded the 

 most complete material for study, and its structural char- 

 acters might accordingly be described, since they are 

 probably common to all members of the group. 



The lateral view of Coccosteus is shown in Fig. 130, the 

 dorsal aspect of the anterior region in Fig. 131, and the 

 ventral view of the visceral region in Fig. 132. It will 

 accordingly be seen that the general shape of the body 

 of this Arthrodiran was somewhat depressed ; that the 

 head, shoulder, and stomach regions were protected by 

 bony plates ; and that the trunk region was lacking in 

 armouring, and short in relative length. In well-preserved 

 fossils the space occupied by the notochord, N, is seen to 



