38 CEREBRAL DEVELOPMENT 



that system no remains of the cranium :* the Silurian fiahcH 

 seem, as has been already said (page 27), to have been exclu- 

 sively placoid, and the purely cartilaginous box, formed by 

 nature for the protection of the brain in this order, has in no 

 case been preserved. Teeth, and, in at least one or two in- 

 stances, the minute jaws over which they were planted, have 

 been found, but no portion of the skull. We know, however, 

 that in the fishes of the same order which now exist, the 

 cranium consists of one undivided piece of a cartilaginous 

 substance, set thickly over its outer surface with minute poly- 

 gonal points of bone (fig. 7), composed 

 internally of star-like rays, that radiate 

 from the centre of ossification, and that 

 present, in consequence, seen through a 

 microscope, the appearance of the poly- 

 gonal cells of a coral of the genus Astrea. 

 Osseous points of placoid The pattern induced is that of stars set 



cramum.f within polygons. Along the sides or top 



(Mag.twelve diameters.) ^ , . / , . f , , , f 



of this unbroken cranial box, that exhi- 

 bits no mark of suture, we find the perforations through 

 which the nerves of smell, sight, taste, and hearing passed 

 from the brain outwards, and see that they have failed to 

 originate distinct vertebral envelopes for themselves : they 

 all lodge in one undivided mansion-house, and have merely 

 separate doors. We find, further, that the homotypal ribs 

 of the entire cranium consist, not of four, but simply of a 

 single pair, attached to the occiput, and which serves both 

 to suspend the jaws, upper and nether, in their place under 

 the middle of the head, and to lend support to the hyoid and 

 branchial framework ; while the scapular ring we find exist- 

 ing, as in the higher vertebrata, not as a cerebral, but as a 



* This, as the reader knows, is no longer the case, the Cephalaspian 

 cranium being most beautifully preserved. 

 + From the head of Eaia clavata. 



