400 STUDIES FOR STUDENTS 



fishes that no less an authority than Smith-Woodward has placed 

 them among the fishes. In general, the group is distinguished 

 by the fact that the bones are not ossified ; that the paired fins 

 and the lower jaw are absent, and that the anterior part of the 

 body is covered by large, bony plates that are developed in the 

 skin and have no connection with the cartilaginous skeleton. 

 The group is divided into three families : Pteraspidce, Cephalas- 

 pidce, and Pterictliidce. 



Pteraspidce . — This family is confined almost exclusively to the 

 Devonian, the Old Red Sandstone of England and Scotland. It 

 contains the simplest and most archaic forms of the Ostraco- 

 dermi. The anterior part of the body was covered by two large 

 plates, a superior and an inferior, that served as a complete armor 

 for that part of the body. The eyes protruded from openings 

 formed by notches in the adjacent edges of the two plates. 

 The upper plate is sometimes marked by grooves that are sup- 

 posed to indicate the course and distribution of sensory tracts 

 such as are found in the skulls of the shark and many of the 

 more advanced types of fishes. The posterior part of the body 

 was covered by many small rhomboid scales. It is probable 

 that the forms were bottom feeders, and that the common food 

 was the abundant molluscan fauna of the Devonian seas. Pteraspis 

 and Holaspis, from the Devonian of England, and Palceaspis, 

 from the Upper Silurian of Pennsylvania in the United States, 

 are the best known of the family. 



Ceplialaspidcz. — In many respects the members of this family 

 resemble the last, the anterior part of the body is covered by 

 well developed plates, while the posterior portion is protected 

 by rhomboid scales. The size was, in general, smaller than 

 either of the other families, seldom reaching more than a foot 

 in length. The head was large and curiously like that of a 

 Trilobite in external appearance. The anterior edge was rounded, 

 and there were two lateral posterior extensions in a position 

 analagous to that of the genal spines in the Trilobites. The 

 eyes were located near the center of this plate. The scales that 

 formed the protection of the posterior part of the body are 



