346 



A HISTORY OF FISHES 





.n 



of the Anaspids, of a reversed heterocercal type, with the fleshy 

 lobe turned downwards and bearing the fin on its upper edge, is 

 unique among fishes, but such a tail occurs as a transitory 

 structure in the larval stages of the existing Lampreys. 



The flexible body and fusiform shape suggests that the 

 Anaspids must have been good swimmers, and probably lived 

 at or near the surface of the water. Of very diflferent shape 

 are the members of the next group, the Cephalaspida^ occurring 

 in the Upper Silurian and Lower Devonian rocks. These were 

 first discovered in the British Isles, the finest specimens of the 

 characteristic genus Cephalaspis occurring in Forfarshire and 



the west of England, 

 while others are found 

 in Europe and Can- 

 ada. Quite recently 

 remains of these fishes 

 have been excavated 

 in Spitzbergen, and 

 some of these are so 

 .^ -f '^ n k'l/NMl/J ]isi perfectly preserved 



r ' ""''^ '9 n \\^N(n rk ^^^^ ^^ ^^ possible to 



/ ..ccr... ^ m\l\\l(!////U make out their in- 



ternal anatomy al- 

 most as accurately as 

 if fresh specimens 

 were available for 

 dissection. Like other 

 fossil Marsipo- 

 branchs, the Cephala- 



Head of Rhyncholepis compared with that of spids were of smallish 

 T .^.... n.^...... . . nostril ; p., pineal organ. ^^^^^ f^^ ^^ ^^^^ ^^_ 



ceeding a length of a 

 few inches. The head is flattened and covered over with a large 

 bony shield, rounded in front, and with the hinder corners 

 generally produced to form pointed "horns" (Fig. 1230). The 

 body is sheathed in numerous hard, bony plates, not unhke 

 those of the Anaspids, arranged in regular rows, those on the 

 sides being high and narrow. Beneath the head-shield there is 

 an internal skeleton, at least partly composed of bony tissue, 

 and all the blood-vessels and nerves penetrating this or lying 

 in contact with it are contained in hard limy tubes, which are 

 preserved in the fossils. Owing to this wonderful state of 

 preservation of anatomical details, coupled with the immense 



Fig. 124. 



1 

 Lamprey (Lampetra) ; n 



(After Kiaer.) 



