LOWER OLD RED SANDSTONE OF SCOTLAND. 349 



head ; — all the placoids, in short, that expire the water taken 

 in for respiratory purposes through more than one gill- open- 

 ing on each side, possess that disposition of the scapular belt 

 which is its prevailing place in the vertebrata. And from 

 this general tjrpe, the arrangement which, in all fishes fur- 

 nished with but one gill-opening on each side, fixes it to the 

 back of the head, seems to be an aberration. Judging from 

 aU we yet know, the placoids of the Silurian system were 

 fishes in which the aberration did not occur : their analojnies 

 lie not with the Chimerides, but with the Cestracionts ; and 

 so, reasoning from our acquaintance with these fishes as they 

 now exist, — of course under that protest, — against the obscu- 

 rity incident on reasoning from the known to the unknown, 

 to which I have referred, we infer that in their first proto- 

 types, as in the mammals, the birds, the reptiles, the sharks, 

 and the rays, the scapular arch was not attached to the head, 

 but occupied its ordinary place down along the vertebrae. 

 Our earliest examples of misplaced scapular belts or cinctures, 

 — of scapular belts removed upwards, and fixed to the back 

 of the head, — occur in those fossiliferous beds of the Old 

 Red Sandstone in which fishes furnished with gill-covers 

 and single gill-openings on each side first appear. Some of 

 the specimens on the table are perhaps the most ancient ex- 

 amples of this coracoidan bone yet found.* 



We find in Professor Owen's work on limbs some of the 

 profoundest thinking on this subject to which the compara- 

 tive anatomist has yet attained. When spending a happy 



* If both placoids and ganoids had their origin about the same pe- 

 riod, as is most probable, or if ganoids really had the start, as they 

 have now, by a single stage of discovery, this theory of misplacement as 

 a mark of degradation will not hold. We infer from the position of the 

 pectorals in Cephalaspis, immediately under the gill-covers, that it was 

 constituted, in respect of the scapular belt, like other-ganoids. The pas- 

 sage following, — a piece of pure reasoning, irrespective of precedence, — 

 forms nrobably the true solution.— L. M. 



