The Class Ostracophori 571 



Dr. Traquair regards the Heterostraci or most primitive 

 Ostracophores as most certainly derived from the Elasmo- 

 branchs. Other writers have attacked the integrity of the 

 group of Ostracophores, questioning the mutual relationship 

 of its component parts. Reiss, for example, regards the asso- 

 ciation of the Osteostraci with the Heterostraci as " unbegriindet " 

 and "unheilvoll," while Ray Lankester, as quoted by Traquair, 

 affirms that "there is absolutely no reason for regarding Cepha- 

 laspis as allied to Pteraspis beyond that the two genera occur 



FIG. 355. Head of Odontotodus schrencki Pander, from the side. (After Patten.) 



in the same rocks, and still less for concluding that either has 

 any connection with Pterichthys." Elsewhere Lankester states 

 that the Heterostraci are associated at present with the Osteo- 

 straci, "because they have, like Cephalaspis, a large head-shield, 

 and because there is nothing else with which to associate them." 

 Patten, on the other hand, seems inclined to deny the rank 

 of Heterostraci and Osteostraci as even separate orders, regarding 

 them as very closely related to each other as also to their sup- 

 posed spider-like ancestors. 



But the consensus of opinion favors the belief that the 

 four orders usually included under this head are distinct and 

 at the same time are really related one to another. For our 

 purposes, then, we may regard the Ostracophori as a distinct 

 class of vertebrates. By placing it after the Elasmobranchs 

 we may indicate its probable descent from a primitive shark- 

 like stock. 



On this subject Dr. Dean remarks: "The entire problem 

 of the homology of the dermal plates and ' scales ' in the Ostra- 

 cophores and Arthrognaths is to the writer by no means as 

 clear as previous writers have conceded. From the histologi- 

 cal standpoint, admitting the craniote nature of the vaso- 

 dentine and cancellous layers in the dermal plates, it never- 

 theless does not follow that they have been derived from the 



