290 THE DERMAL SKELETON. 



ostracoderms, and consists of a practically continuous bony envelop for the head 

 and gill region, with segmentally arranged plates, loosely articulated, covering the 

 joints of the trunk and tail. Such an exoskeleton resembles that of the marine 

 arachnids, the hypothetical ancestors of the vertebrates. 



It is assumed that the cephalic buckler of the ostracoderms corresponds to the 

 cephalothoracic shield of the merostomes and arachnids generally, i.e., the primi- 

 tive head, the first six thoracic, and the vagus segments. The buckler of such 

 forms as Pteraspis, Tremataspis and Bothriolepis also includes the abdominal or 

 respiratory segments. 



The line of union between the thoracic and branchial regions is clearly indi- 

 cated in Bothriolepis by the hinge-like joint separating the cephalic buckler from 

 the respiratory chamber. (Fig. 247.) Just in front of this joint are two pores 

 leading into the interior of the shell. Two similar pores are found in Tremataspis 

 (Fig. 236), and just back of these pores is probably the dividing line between the 

 cephalic and branchial regions in these animals. 



In Cephalaspis and Tremataspis, the trunk region is covered with segmentally 

 arranged dermal plates corresponding with the plates covering the neural surface 

 of the post-branchial region in arthropods. Only a few of the more anterior 

 segmental plates of the ostracoderms are represented in the arthropods, the more 

 posterior ones being new formations added after the separation of the vertebrate 

 from the arthropod stock had taken place. 



In Bothriolepsis, and in a small undescribed species of Cephalaspis from 

 Dalhousie (Fig. 234), the trunk and tail are naked, save for a few minute, irregu- 

 larly distributed ossicles near the anterior end of the trunk. 



In Pterichthys and Pteraspis, the trunk appears to have been covered with 

 rounded or polygonal scales, probably formed by the breaking up of larger seg- 

 mental scales like those of Trematapsis. 



The Minute Structure of the Dermal Bones of the Ostracoderms. 



The structure of the dermal bones of Tremataspis, Pteraspis, and Bothriolepis 

 has been studied with special care, and some observations were made on the 

 dermal skeleton of Cephalaspis and Tolypaspis, but they are incomplete, owing to 

 the lack of adequate material. 



It is surprising how beautifully the details of the minute structure of these 

 ancient fish bones is preserved. When properly prepared, the color and the 

 minutest details may occasionally be seen as clearly as though the animals had 

 been dead a few weeks only, instead of a few million years. 



Tremataspis. 



In Tremataspis the outer surface of the shell generally has a light yellowish- 

 brown color. It is beautifully polished and under the Jens appears to be orna- 

 mented with low winding ridges and mounds, similar to those of Bothriolepis, 



