vi. 9 PLACODERMS 187 



It has already been mentioned that the earliest gnathostome verte- 

 brates found in the rocks do not have the shark-like form, and present 

 so many peculiarities that they are placed in a distinct class. In the 

 past the fossils included here have been referred to various groups, 

 usually either to the agnatha or the elasmobranchs, and there is still 

 some doubt as to their position. In many respects they are highly 

 specialized, but they all have one feature that may be presumed to 

 have existed in the ancestral gnathostome, namely, that the hyoid arch 

 played no part in the support of the jaws and the spiracle was there- 

 fore a typical gill-slit. For this reason they are often given the name 

 Aphetohyoidea, but we shall prefer to call them Placodermi, to em- 

 phasize that they all have a heavy armour of bone-like material. The 

 class contains several orders, not obviously very closely related to 

 each other; all are fossil forms, none of which is known to have sur- 

 vived the Permian. 



The best-known, earliest, and perhaps most interesting group is 

 the acanthodians, found in freshwater deposits extending from the 

 Upper Silurian to the Permian but chiefly in the Devonian. These 

 were small fishes with a fusiform body (Fig. 115), with heterocercal 

 tail and two, or later one, dorsal fins. The lateral fins consisted of a 

 series of pairs, often as many as seven in all, down the sides of the 

 body. The effect of these in stabilizing the fish would presumably be 

 different from that of a continuous fold, and the problem of the form 

 and function of the earliest paired fins remains obscure. The fins were 

 all supported by the large spines from which the group derives its 

 name. 



The whole surface of the body was covered with a layer of small 

 rhomboidal scales, composed of layers of material ressembling bone, 

 covered with a shiny material similar to the ganoin of early Actino- 

 pterygii. On the head these scales were enlarged to make a definite 

 pattern of dermal bones, numerous at first but fewer in the later forms. 

 The pattern of the bones has no close similarity to that of later fishes. 

 The reduced bones of the later acanthodians are related to the lateral 

 line canals, which have an arrangement similar to that in other fishes, 

 but run between and not through the scales and bones of the head. 

 The teeth are formed as a series of modified scales. The skull is partly 

 ossified — important evidence that the boneless condition of elasmo- 

 branchs was not typical of all early gnathostomes. 



The jaws of acanthodians were attached by their own processes to 

 the skull (autodiastyly) and are remarkable in that four separate 

 ossifications take place in them (two in the upper and two in the lower 



