190 



ACANTHODII 



and contains numerous cells which may have branching processes. 

 The marginal teeth of the jaws, sometimes large and multicuspid, 

 are firmly fixed to these 'bones' in some genera (Fig. 160). 

 As a rule, the teeth are small, or absent. Sometimes, however, 



Vm. 150. 



Diagram of the skull and visceral arches of an Acanthodian. l>r, fifth branchial arch ; 7im, 

 hyomandibular ; h.p, calcified plate bearing similar branchial rays (?) ; m, Meckel's cartilage ; 

 o.p, otic process of the palato-quadrate ; po, ethmoid process. 



they are of considerable size, and Dean has described in Ischna- 

 canthus rows of teeth, with large central and small lateral cusps, 

 resembling those of primitive sharks (Cladoselachians, etc.). The 

 ceratohyal seems to be covered with a plate bearing a series (or 

 two series, Eohon [367]) of long branchial rays, all of similar hard 



st 



FIG. 160. 



Portion of lower jaw of Isehnacanthus, L. Devonian, Forfar ; outer view. 

 b, bone-like tissue ; st, inner teeth ; t, marginal tooth. 



tissue or of dentine (A. S. Woodward [505], Eeis [352]). Long 

 branchial rays also occur on the branchial arches ; but whether in 

 single or in double series is not yet clear. Perhaps more peculiar 

 still is the structure of the pectoral girdle. It remains in some of 

 the fossils (Acanthodes, Fig. 162), in the shape of a rod fixed to the 

 base of the spine; in others (Parexus, Fig. 161), a broad clavicular 



