FOSSIL VERTEBRATES — FISHES 4° I 



large and arranged in rows, the mid-dorsal row developing an 

 acuminate ridge that has the appearance of a dorsal fin. The 

 tail was distinctly heterocercal. In some of the more perfectly 

 preserved specimens there seems to be an indication of the 

 presence of external gills at the base of the posterior lateral 

 spines of the head plate. All the known forms are from the 

 Upper Silurian and the Devonian rocks of England and Europe. 

 Among the best known of the genera are, Ccphalaspis, Anchen- 

 aspis and Tremataspis 



Ptericthidce. — This family presents many important steps in 

 advance of the other two, the anterior part of the body is not 

 protected by single plates, but by an armor made up of the 

 union of several small plates both upon the upper and the lower 

 sides. The posterior portion was, as in the other forms, covered 

 with small scales. Perhaps the most peculiar thing about the 

 family is the presence upon the sides of the body, near the 

 anterior end, of elongate, movable appendages that perhaps 

 served as swimming organs, although in one of the later and 

 more specialized of the forms the appendages become anchylosed 

 to the adjoining plate, and lose all power of motion. These 

 appendages are regarded as the homologues of the posterior 

 extensions of the head plate of the Cephalaspidce. The Pteric- 

 thidce are most commonly known from the Devonian of the Old 

 World. Ptericthys, Asterolepis, and Bothriolepis are well-known 

 European forms. From the New World Bothriolepis has been 

 described from the Devonian of Canada, and from the same 

 horizon in Ohio incomplete remains have been described by 

 Newberry as Acanthaspis and Aca?itholepis. It is necessary here 

 to warn the student against a confusion that may arise between 

 the old classification present in so many of the text-books 

 and the one here used. The Pteraspidce, Cephalaspidce, and the 

 Placodermi were regarded as orders of the highly artificial group, 

 Ganoidei. The last order included not only the Ptericthidce, 

 but more highly developed forms that are now known to belong 

 to the Dipnoi. 



The Elasmobranchii are the most primitive forms that hold an 



