338 AN INTRODUCTION TO THE STUDY OF FOSSILS 



of an external (dermal) armor (whence the name from Greek 

 ostrakon, shell, + derma, skin). There is always a large shield 

 composed of several pieces, completely inclosing the head and 

 usually also the thorax. The tail was flexible and usually pro- 

 tected by scales. A pair of dorsally or laterally placed eyes was 

 present ; between the orbits is a deep pit, at times only visible 

 upon the inner surface of the shield, occupying the exact posi- 

 tion of the pineal body of the vertebrate brain. These animals 

 had probably the mud-grubbing habit like the contemporaneous 

 eurypterids and the living horseshoe crab. 



Fig. 143. — Restoration of the ostracoderm, Cephalaspis murchisoni, from the Upper 

 Silurian of England, c.f., caudal fin (this is heterocercal) ; d.f., dorsal fin; h.s., 

 head shield ; ob., orbit of eye. (From British Museum Catalog.) 



The Ostracodermi are well known from the Ordovician 

 through the Devonian of North America and Europe. They 

 are divided into the two groups, Euostracophori and Placoder- 

 mata. Under the former are included the most primitive forms 

 {e.g. Cephalaspis of the Upper Silurian and Devonian), with a 

 head shield only (Fig. 143). In the Placodermata are placed 

 the more complicated forms (such as Pterichthys of the Devo- 

 nian), with a shield, composed of several overlapping plates, sur- 

 rounding head and trunk, and Dinichthys, a huge armored 

 genus from the Devonian of North America and Europe, whose 

 head shield at times measures over three feet across. 



1. Define Class Ostracodermi. 



2. To what phylum other than the Chordata may these 

 fossils belong ? 



3. What is their age geologically? 



4. What were their probable habits? 



