nasal openings 



parietal foramen 

 eye 



sensory areas 



B 



mouth 



orobronchiol plates 

 branchial opening 



endolymphatic foramen 



posterior margin of 



endocranium' 



sensory lines (dashed) 



dorsal fin 



anal plate 



dorsal crest or dorsal fin 



lateral crest 



anal plate 



Figure 5-30. Cephalothoracic armor of rremofospis mitleri, an osteostracan. A, lateral view with 

 main sensory lines indicated; B, dorsal view; C, ventral view; D, posterior view showing dorsal and 

 lateral crests. (After Westoll, 1958) 



joined anteriorly and bearing the ethmoid tooth; blastemas 

 for Meckel's cartilages, extending downward from the pos- 

 terior ends of the palatoquadrate condensations; and a true 

 hyoid arch, inside the posterior margin of the subocular 

 arch. The Meckel's cartilages later join to form the dental 

 plate. The tongue base could represent an external series of 

 elements related to Meckel's cartilages and/or the longitu- 

 dinal elements joining the ventral ends of the visceral arches. 

 Comparisons of the lamprey and hagfish reveal some 

 similarity but not the sort of agreement seen among the 

 gnathostomes. Holmgren and others have drawn many par- 

 allels between agnath and gnathostome, but these at best 

 are tenuous. The rasping tongues of both have been gen- 

 erally accepted as proof of common ancestry; yet in details 

 these structures differ in the two types. 



Fossil agnaths 



The agnaths extend back in time to the Middle Ordovi- 

 cian. Those from that period are represented by bony bits of 

 plates and occasionally larger imbricated areas. It is not 

 until the Upper Silurian and Devonian that much is known 

 of their structure. Three basic types have been described: 

 heterostracans, osteostracans, and anaspidans (Figure 5-29). 

 The best known are the osteostracans. 



Osteostracan The osteostracan head skeleton was fused 

 with the trunk armor, which extended variable distances 

 back over the body to form a carapace. The earliest osteo- 

 stracans were the tremataspids of the first part of the Upper 

 Silurian (Figure 5-30). In these the solid carapace was inter- 



AGNATH FISHES 



135 



