ADAMS, PHYLOGEXY OF THE JAW MUSCLES 121 



stome" stage of evolution and probably represent the forerunners of the 

 ganoids and Tetrapoda, while possibly having remote relationships also in 

 another direction with ancestral elasmobranchs. . . . The Antiarchi have 

 advanced beyond the typical Ostracoderms in having the head sharply differ- 

 entiated from the thorax and the mouth armed with functional jaws, which 

 are fashioned from the dermal skeleton. But not even the exquisitely pre- 

 served specimens of Bothriolepis described by Patten (1912) show any traces 

 of the cartilaginous jaws, branchial arches or cartilaginous axial skeleton. 



In another passage this author writes : 



The upper Silurian Birkenia of Traquair apparently had no biting jaws and 

 may have sucked in small particles of food like the larval lamprey. Well 

 preserved material showed that none of the Ostracoderms had cartilaginous 

 jaws or teeth, but the dermal plaques around the oral hood sometimes func- 

 tioned as jaws. Typically carnivorous habits, involving true cartilage jaws, 

 true teeth, and both paired and median fins, are first shown in the Acanthodian 

 sharks of the upper Silurian and Devonian. 



We may therefore assume that cartilaginous jaws first appeared as such 

 in the true fishes (elasmobranchs). In the Silurian and Devonian ostra- 

 coderms there is a progressive series that shows some of the many attempts 

 to produce workable jaws. The Anaspida, though fish-like in form, have 

 progressed but slightly toward the true fishes. Lasanius had some struc- 

 tures back of the poorly formed head that suggest dermal gill supports. In 

 certain Heterostaci (Thelodus and Lanarlcia) the skin was covered with 

 denticles that resemble those of elasmobranchs. These denticles would 

 strengthen the skin and give the muscles of the skin fascia much better 

 support. The Drepanaspidae show marked progress in the strengthening 

 of the skin by the formation of plates of different sizes. Drepanaspis and 

 Pteraspis show a differentation in the head region, but it is more to be 

 compared to the hard covering of some insects, as the cartilaginous sup- 

 ports of the interior were feeble or lacking. The mouth was unlike that 

 of either elasmobranchs or teleosts, but seems to have been a wide slit 

 stiffened by dermal scutes. The Ostrastraci show more specialization 

 along this same line, but with no better results. The. clumsy plates of 

 Ceph alas pis, Tremataspis, etc., give little promise of anything that would 

 be much better than the arthropod jaw. The placoderm fishes of the order 

 Antiarchi made a more clearly defined attempt to have movable jaws 

 formed from dermal plates. A study of the fossils shows that they had a 

 mouth that was functionally analogous to the mouth of fishes — that is, 

 their dermal jaws correspond in position, but are not homologous with 

 dermal premaxillse, maxillae and mandibles. The head is slightly movable 

 on the shoulder, as in Arthrodira, and the jaws could apparently move 

 laterally as well as vertically. Patten (1912) holds that they were bottom- 



