THE VERTEBRATES. 



presence of rudimentary appendages on the oral and branchial arches, by a small 

 mouth with a feeble lower jaw, short body cavity and slender tail, and by the 

 absence of postbranchial paired appendages. This tadpole larva is clearly the 

 recurrence of the ostracoderm stage in their phylogeny. (Fig. 167.) 



The Arthrodira. — The arthrodires closely resemble the ostracoderms in the 

 structure of their jaws and in the arrangement of their cranial plates, and without 

 doubt are directly descended from them. While it is not possible to identify 

 in detail all the various structures involved in the general resemblance that runs 

 tjirough them all, we can trace a progressive series of structures and events that 

 lead steadily upward from the ostracoderms, through the arthrodires, to the 

 vertebrates. 



Without entering into a detailed discussion of the arthrodires, an examination 

 of Coccosteus, a fairly well-known and typical representative, will show us the 

 more important respects in which they approach the vertebrates. 



In Coccosteus (Figs. 260, 263), the central aggregate of procephahc sense 

 organs seen in the ostracoderms has separated. The lateral eyes have increased 

 greatly in size and have taken up an antero-Iateral position, losing apparently 

 some of their dermal armor and their power to rise and fall in the orbits; the 

 parietal eye is lodged in a small median plate, located in about the same posi- 

 tion as before, while the olfactory organs have moved forward and laterally, 

 occupying a more nearly terminal position. 



Jaws. — Three pairs of jaws are present. The premaxillae are relatively 

 smaller than in the ostracoderms, and although still distinctly paired, are less freely 

 movable in a transverse direction. The rudiments of toothed maxillae are present, 

 for the first time, as small free plates that probably represent the small ventro- 

 lateral plate of Bothriolepis. The mandibles have increased greatly in size and 

 may be provided with prominent tooth-like spikes on their anterior and median 

 borders, being in this respect more like arthropod jaws than those of Bothriolepis, 

 the only ostracoderm whose mandibles are known. Like the ostracoderm man- 

 dibles, they were capable of very complex movements. Both ends were free; 

 that is, they were not firmly articulated to any cartilage or bone, and could be 

 either rotated, or moved in a transverse and longitudinal direction. Their ex- 

 posed median ends were probably held in place by the integument, while their 

 lateral ends were buried in the tissues of the head, and served for the attachment 

 of the sinews and powerful muscles that controlled their movements. 



A, single hyoid arch (Fig. 260, h), covered with dermal bone, extended 

 across the throat behind the mandibles, in place of the two arches of similar 

 structure seen in Bothriolepis. 



The cranial bones are more numerous than in the antiarcha, owing probably 

 to the breaking up of the large orbital plate by the lateral migration of the eyes. 

 The same movement has probably opened the way for the formation, for the 

 first time, of a supra-orbital line of cutaneous sense organs. 



The branchial shield retains nearly the same number and arrangement of 



