GREGORY: FISH SKULLS 



77 



The agnathous ostracoderms of the Silurian period, taken as a class, reveal to us what 

 those primitive characters were, even though they were perhaps not the characters that 

 we expected. For as a class the ostracoderms are so inferior to the gnathostomes in their 

 locomotor apparatus that they have even been assumed to be a specialized bottom-living 

 group with no claim to be considered in the line of ascent to the gnathostomes. That was 

 partly because it was further assumed that the continuous "headshield" must always be 

 the result of the fusion of small polygonal plates. But Stensio's intensive researches have 

 revealed that the primitive ostracoderm shield was supported by a continuous endoskeleton 

 without sutures, which was covered by a bony membrane. 



naso-/,ypophyseai pj^eal opening 

 opening \ ' ■ ' 



orobranrhiaJ 

 chamber 



C 



fg*?A^'' notochord 



oesophagus 



roof of mouth ca vity 



muscle of 

 .gillarch 



r external 

 \ duct of 

 I ^ill sacK 



Fig. 2. Relations of the orobranchial chamber to the neurocranium in ostracoderms and cyclostomes. 



A. Tentative restoration by Stensio of the ventral aspect of the head of a cephalaspid, showing probable position of mouth 

 and gill openings. 



B. Roof of orobranchial chamber of cephalaspid, with gill-tubes, gill-clefts and interbranchial partitions, the latter extending 

 downward from floor of neurocranium. Restoration by Stensio. 



C. Schematic median sagittal section of cephalaspid head-shield showing relations of neurocranium and orobranchial 

 chamber. Exoskeletal bone in thick lines; perichondral bone layers in fine lines. After Stensio. 



Differentiation of the Neurocranium and the Branchiocranium 



The capacious orobranchial chamber of ostracoderms (Fig. 2) lay immediately beneath 

 the brain and the cranial nerves and vessels; but there was no sharp separation between 



