rupted above by openings for the eyes and by the pineal 

 nasal organs. The nasal opening lay just anterior to the eyes; 

 it was partly divided into anterior and posterior openings in 

 the cephalaspids (Figure 5-31). 



Behind the eye openings was a depression, closed above 

 by a mosaic of small plates; this was the dorsal field. To 

 either side were similar anterior and posterior lateral fields. 

 Posteriorly the trunk part of the shield had a dorsal crest. 

 Laterally the shield had small folds. 



Ventrally, the mouth and pharyngeal area was open. 

 The lateral and posterior margins of this ventral opening 

 had ten notches, decreasing in size from in front backward, 

 for the outer branchial openings. This area was covered in 

 life by a mosaic of small plates, except for the mouth and 

 branchial openings. The T-shaped mouth was in the ante- 

 rior part. 



The endocranium and visceral skeleton of the tremataspid 

 is not well known but was probably like that of the more 

 advanced cephalaspids, which have been described in detail 

 (Figure 5-31). The endocranium and visceral skeleton were 



indistinguishably fused with each other and with the dermal 

 carapace. They were in part identifiable as only perichon- 

 drally ossified. The degree of ossification varies from .solid to 

 superficial. The cranial cavity opened anteriorly into the 

 nasal cavity, which opened to the outside. The inner ear 

 was enclosed in a bony capsule, and, from the vestibular 

 cavity, a number of canals radiate to the dorsal and lateral 

 sensory fields. The walls of the cranial cavity were pene- 

 trated by many nerve and vascular foramina. The seventh 

 nerve passed through the vestibular cavity. The bone of the 

 endocranial margin was penetrated in the later forms, but 

 not the tremataspids, by large venous sinuses. The notochord 

 was much constricted, lying generally below the basis cranii 

 in a groove but penetrating into the dorsum sellae. The tip 

 of the notochord was not exposed in the sella. The endo- 

 cranium extended posteriorly above the notochord into the 

 root of the dorsal crest of the carapace, and it also extended 

 back along the lateral margin. 



The roof of the oral cavity was ridged between the gill 

 pouches, of which ten pairs are generally indicated (11th 



lateral field 



orobronchiol plates 

 nasal openings 

 parietal foramen 



dorsal field 



mouth orobronchiol cavity 



branchial opening 



dorsal aorta 

 hyoid arch_ 



aortic arch (4' 

 esophagus- 

 ventrol aorta 



brain cavity 



esophagus and ventrol aorta 



Figure 5-31. Cephalothorocic ormor of cephalaspids and osteostracans. A, dorsal view of 

 Kierospis, B, ventral view with plates of orobronchial area and pectoral paddle as seen in Hirel/a, C 

 ventral vUw with orobronchiol plates removed to show contours of bony roof of mouth; D, lateral 

 view; E, sagittal section as seen in Mimefospis. (After Stensio, 1958) 



136 • THE HEAD SKELETON OF FISHES 



