GNA TH OS TOMES. 



225 



ORDER II. ASPIDOCEPHALI 

 (OSTEOSTRACI). 



Head covered by a large, simple, arcuate 

 shield much like the cephalothorax of Limiilus 

 in outline, with orbits ' near its centre. Tail 

 covered with rhomboid scales of varying size. 

 \> paired appendages known. Cephalaspis, 

 Devonian. Aitchcnaspis, upper Silurian. 



ORDER III. ANTIARCHA. 



Head and trunk covered with large polygo- 

 nal plates coated with enamel ; tail with small 

 scales, ' orbits ' dorsal, close together, a single 

 pair of appendages ( pectoral fins'), covered 

 with strong plates, and each jointed near the 

 middle. Pterichthys (lower Devonian), 1-4 

 inches long. Asterolepis, 6 inches long (Devo- 

 nian). BotJiriolepis. 



M 



FIG. 230. Cephalaspis, 

 from Dean, after Agassiz. 



FlG. 231. Restoration of Pterichthys testudinarius, after Traquair. ADL, 

 anterior dorso-lateral ; AMD, anterior median dorsal; AVL, anterior ventro- 

 lateral ; EL, extra lateral (opercular) ; L, labial; MO, median occipital; PDL, 

 posterior dorso-lateral; PM, premedian ; PAID, posterior median dorsal; 

 posterior ventro-lateral ; SI., semilunar. Lateral line system dotted. 



Series II. Gnathostomata. 



Vertebrates with jaws ; paired appendages normally present ; 

 olfactory organs paired, and not connected with hypophysis; 

 optic chiasma present ; three semicircular canals in the ear ; 

 gills, when present, never more than seven pairs. 



The gnathostomes embrace the great majority of verte- 

 brates, and the account of that group given in the first part of 

 this volume applies largely to these forms. The mouth is trans- 

 verse ; paired limbs occur in all, except where, as in certain 



