40 8 STUDIES FOR STUDENTS 



ments. Specimens of this genus are known from the Devonian 

 rocks of the province of Quebec in Canada. Ctenodus, from the 

 Carboniferous of England, Holodus and Palcedaphus from Devo- 

 nian of Europe, My lo stoma from the Devonian of New York, and 

 Gnatlwrliiza and Strigilina from the Permian of Texas, are all 

 characteristic forms that have been described from the teeth. 



A division of the Dipnoi, the Arthrodira are of especial 

 interest, as they were at one time regarded as belonging with 

 the Ptericthidce in an order the Placodermi. The discovery of 

 well-developed lower jaws and paired fins demonstrated that 

 they could not belong among the primitive Ostracodermi, and 

 the discovery of the manner of the articulation of the lower jaw 

 to the skull showed that they belonged among the Dipnoi. They 

 are among the most ancient of the fishes, ranging from the 

 Upper Silurian to the Carboniferous. In the United States a 

 large majority of the known remains have been taken from the 

 Waverly Shales of Ohio. In Europe they are found in the 

 Devonian Old Red Sandstone of England and Scotland. They 

 were powerful, armored, predatory forms, in many cases of large 

 size, that must have been a match for the largest sharks of the 

 time. The armor in some of the genera was between two and 

 three inches in thickness. The armor seems to have been con- 

 fined in most of the forms to the anterior portion of the body, 

 which has led to the belief that perhaps they buried the pos- 

 terior part of the body in the mud and lay in wait for their prey 

 rather than seeking it out and depending on their rapidity of 

 movement and powerful jaws to obtain the mastery. 



Coccosteus, a rather small form from the Devonian of Eng- 

 land, is one of the best known of the group. It did not reach 

 a length of more than one or two feet. The anterior part 

 of the body is covered with an armor made up of several 

 plates that extend back to about the middle of the body. The 

 centra of the vertebrae are not preserved but the upper and 

 lower arches outline the notochordal column. The dorsal fin is 

 in the stage of the basals and radials and the posterior pair of 

 fins is united to a distinct pelvic girdle. The armor of the 



