Xlviil PREFATORY REMARKS 



point which occasions the disagreement of fossil anatomists 

 as to their actual standing. In point of brain, they are 

 not so highly organized as the placoids, their immediate suc- 

 cessors ; but it is only in the race of discovery that these 

 placo-ganoids have gained the advantage by a single stage ; 

 and presumptive evidence favours the supposition that placoid 

 and ganoid life took their rise together. How far back into 

 the Silurians this may have been, it is as yet impossible to 

 say. It may be mentioned, by the way, that too many genera 

 were originally erected out of the placoid remains found in 

 the Upper Ludlow bone-bed. A few, too, have proved spu- 

 rious ; but no doubt whatever is entertained regarding some^ 

 accompanied as they are by pieces of shagreen, which prac- 

 tised eyes, when assisted by the microscope, cannot fail to 

 distinguish from the outer dermal coverings of the plates of 

 ganoid fish or crustaceans. 



The reader already acquainted with the "Foot-prints" 

 may have made another observation on some of the opinions 

 we have submitted to him, viz., that the principle embodied 

 in the chapter entitled "Rank Dependent on Brain, not 

 Bone," bids fair to be very generally adopted. Twelve years 

 ago, when the " Foot-prints" was written, the author had no 

 authority to fall back upon. But now, as we have seen, Pro- 

 fessor Huxley considers it an essential point in the classifi- 

 cation of fishes, and Professor Owen has boldly adopted it 

 in regard to the mammalia, though he extends it for the pre- 

 sent no farther. The mammalia he distinguishes as perfect- 

 brained, COL olute-brained, smooth-brained, loose-brained;* 



* See the Eev. W. S. Symonds' recent work, before mentioned, id 

 which Owen's system is admirably condenseti. 



