590 Arthrodires 



as far as the present evidence goes, pterygial, nor are the similar 

 structures in Brachydirus.* The sigmoid element, described 

 as a 'pelvic girdle' by Smith Woodward, in Coccosteus, a struc- 

 ture which appears to occur in a small species of Dinichthys(!) , 

 may as reasonably be interpreted as a displaced element of 

 the armor-plates of the trunk. In Coccosteus, as far as I am 

 aware, it occurs in well-preserved condition in but a single 

 specimen. 



" In referring to the singular joint between the shoulder- 

 plates and the hinder margin of the cranium Smith Woodward 

 has called attention to one of the striking features of the group. 

 It is one, however, which, as a functional structure, i.e., a joint, 

 characterizes only a portion of its members; and in these the 

 region in which vestiges of the joint are sought is overlaid and 

 concealed by dermal plates. Such are the conditions in Macro- 

 petalichthys (with transitional characters in Trachosteus and in 

 Mylostoma). For this form a special subclass (or order) may 

 be created which we may term Anarthrodira. 



"SECOND. The A rthrognathi cannot well be included in any 

 other class. It would certainly be more convenient to retain 

 the Arthrognaths among the Ostracophores, regarding them 

 as a fourth subclass, were it not that they differ from them in 

 so marked a way in the presence of well-marked vertebral arches, 

 of supports for the unpaired fin, and in the possession of ' jaws.' 

 In these regards add to them the (probable if not certain) 

 absence of the paired paddle-like ' spines ' they stand certainly 

 further from the Antiarcha than these from the Osteostraci, 

 or than the latter from the Heterostraci. It appears to me 

 desirable, therefore, that the Arthrodira and the Anarthrodira 

 be brought together as a separate class. Should subsequent 

 researches demonstrate a closer affinity with the Ostracophores, 

 the Arthrognathi can be regarded as of rank as a subclass, with 

 the orders Anarthrodira and Arthrodira." f 



* It is by no means impossible that there may ultimately be found pectoral 

 elements to correspond in a general way with the paddle-like "spines" of 

 the Antiarcha. 



t The group Placodermi, created by McCoy (1848) as a "family" for the 

 reception of Coccosteus and Pterichthys might then be justly elevated to 

 rank as a class, superseding the Ostracophori of Cope (1891). The latter 

 group might, however, be retained as a subclass, and include the Hetero- 

 straci and Osteostraci as ordinal divisions. 



