56 EXPLANATIONS. 



is in general by corresponding apertures on the 

 sides of their neck ;* other fishes having free 

 gills, marking a higher organization. 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 families of several of the higher orders of 

 the vertebrata. Thus, the cestraceon is, on many 

 considerations, alow fish, though certainly possess- 

 ing some traits of superior character, and not 

 the lowest of its order. In the second place, I 

 would protest against any inference unfavourable 

 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 iVymestry limestone, in the 

 Upper Silurians. t Now, we have a cestraceon an- 



* Fletcher's Physiology. Part 1, p. 20. 

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

 Ludlow rock, are the earliest precursors of many singular ichthy- 



I 



