10 



AGNATHA. 



middle line, is conspicuous in advance of the orbital region. As shown 

 from beneath (fig. 8), the margin of the shield is reflexed inwards to 

 form a flattened and ornamented inferior rirn, wider behind than in 

 front; and, as proved by transverse sections (fig. 9), the inner border 

 of this rim is continued upwards into a delicate smooth lamina of calci- 

 fied tissue (i), which lies beneath the outer or superior lamina () of the 

 shield proper. At each postero-lateral angle of the shield, the middle 



Cephalaspis; diagram of lower aspect of dorsal shield, showing inferior rim. 

 (After Lankester.) 



Cephalaspis ; transverse section of dorsal shield, i, inferior lamina ; m, margin ; 

 s, superior lamina. (After Lankester.) 



vascular layer of its substance is continued backwards as a flexible flap, 

 which is sometimes nearly half as long as the shield (fig. 10). This is 

 probably related to the outlet for the water from the branchial chamber. 

 Nothing is known of the endoskeleton. There is one small remote dorsal 

 fin, and the tail is heterocercal ; but no well-defined fin-rays can be ob- 

 served, the supporting structures of the fin-membrane being small, oblong, 

 calcified plates, closely fitted together, and placed end to end in vertical 

 parallel series. The earliest known fragment of Cephalaspis was obtained 

 from the Ludlow Tilestones, while fine examples of C. murchisoni (fig. 10) 

 occur in the Passage Beds immediately below the Old Red Sandstone at 



