GNA THOSTOMES. 



225 



ORDER II. ASPIDOCEPHALI 

 (OSTEOSTRACI). 



Head covered by a large, simple, arcuate 

 shield much like the cephalothorax of Limulus 

 in outline, with 'orbits' near its centre. Tail 

 covered with rhomboid scales of varying size. 

 No paired appendages known. Cephalaspis, 

 Devonian. Auchenaspis, 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). Bothriolepis. 



M 



FlG. 230. Cephalaspis, 

 from Dean, after Agassiz. 



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

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

 lateral ; EL, extra lateral (opercular); /., labial; AfO, median occipital; PDL, 

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

 posterior ventro-lateral ; SL, 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 



