FOSSILS OF OLD RED SANDSTONE. 



229 



the older strata ?" Now I cannot tell what good natural- 

 ists may say in answer to this appeal ; but I feel, for my 

 own part, that the facts in question — as far as they can 

 be admitted to be so — have no such destructive effect. 



In the first place, the cestraceon is only one of Ihose 

 cartilagines, the real character of which had just been 

 explained. It is not the lowest of its order, but neither 

 is it the highest. So far from this being the case, the 

 respiration of the whole family (Selacii, Cuv. ; Plagios- 

 tomi, Desm.) to which it belongs, and which also includes 

 sharks, is performed in a manner which approximates 

 these fishes to the worms and insects — namely, 64 by 

 numerous vesicles called internal gills, the entrance to 

 which is from their gullet, while the exit is in general 

 by corresponding apertures on the side of their neck ; " * 

 other fishes having free gills, marking a higher organiza- 

 tion. The sub-divided form of the stomach — the absence 

 of that concentration, which is, perhaps, the most em- 

 phatic mark of animal advancement — belongs to this 

 family alone amongst fishes, as it does to the lowest fami- 

 lies of several of the higher orders of the vertebrata. 

 Thus, the cestraceon is, on many considerations, a low 

 fish, though certainly possessing some traits of superior 

 character, and not the lowest of its order. In the second 

 place, I would protest against any inference unfavorable 

 to the hypothesis of development being drawn from a 

 discovery so new, so isolated, and in a branch of inquiry 

 so extremely unsettled. At no time during the last ten 

 years have we had, for a twelvemonth at once, stable 

 views respecting the initiation of fishes. Lately — so 

 lately that part of my book was written at the time— the 

 lowest were understood to be some of a minute size, im- 

 mediately over the Aymestry limestone, in the Upper 

 Silurians, f Now we have a cestraceon announced to us 

 at a lower point in that formation. But how far it is 

 likely that our information is to rest at this point the 

 reader may judge, when he hears of M. Agassiz an- 

 nouncing, within the last few months, that, though ac- 

 quainted with seventeen hundred species of fossil fishes > 

 * Fletcher's Physiology, part i., p. 20. 



f " The minute and curious fishes in the uppermost bed of the 

 Ludlow rock are the earliest precursors of many singular ichthy- 

 olites which succeed in that enormous formation the Old Red 

 Sandstone."— Murchison's Address to the Geological Seciety, Feb 

 mary, 1S42. 



