1 88 EVOLUTION OF ELASMOBRANCHS vi. 9 



jaw), making a series of elements similar to that found in the typical 

 branchial arches. The hyoid was an unmodified branchial arch. At 

 first the mandibular, hyoid, and each of the branchial arches were pro- 

 vided with small flap-like opercula, but in later forms the mandibular 

 operculum became especially developed and covered all the gills. 



These animals might well represent the ancestors of many if not all 

 other groups of gnathostomes. They have not the peculiar features 

 that we characterize as shark-like, and though they may well have 

 been carnivorous they are not very highly specialized for that mode of 

 life. Whether or not the known acanthodians represent the actual 

 ancestors of the other gnasthostome groups, it is clear that knowledge 

 of their anatomy forces us to discard two conclusions which have 

 often been accepted in the past, namely, that lack of bone and an 

 amphistylic jaw support are primitive gnathostome features. Here 

 already in the Silurian we find animals that possessed both endo- 

 chondral bone and scales composed of bony substance. Moreover, 

 some of them have no trace of denticles and we must therefore regard 

 with suspicion any theory that considers the placoid scale as the 

 original type of all scales. It is at least as likely that scales composed of 

 simple layers of bone in the dermis were the ancestral type and that 

 placoid forms with a pulp cavity were a later specialization. 



Several other types of placoderm fish are known, mostly from the 

 Devonian strata. The Arthrodira, Macropetalichthyida, and Anti- 

 archi (Fig. in) were mostly heavily armoured fishes with dermal 

 bones on the head and often a large shield over the body. There was 

 usually a heterocercal tail and a covering of scales. The earlier fishes 

 were mostly from fresh water, the later from the sea. Many were 

 rather flattened, probably bottom-living and invertebrate-eating 

 forms. The bony plates on the head were often arranged in charac- 

 teristic patterns, none of which, however, shows close similarity to 

 the pattern of bones on the head of bony fishes or tetrapods. Lateral 

 line canals of typical arrangement were present and the 'bones' follow 

 these to some extent. 



*Gemundina was a flattened animal, superficially similar to a skate, 

 from marine Lower Devonian deposits. The skin was covered with 

 denticles, but under these were large plates, apparently of bone. This 

 fish is placed in a special order Stegoselachii and its affinities are 

 unknown, but it shows again that the tendency to develop a flattened 

 form has been present from the earliest appearance of fishes. *Palaeo- 

 spondylus from the Devonian is another isolated form, in the past often 

 classed with the cyclostomes. Moy-Thomas showed, however, that 



