50 EXPLANATIONS. 



makes a strong point of the placoid and ganoid 

 orders, as unfavourable to the progressive theory. 

 " Taking into account," he says, " the brain, and 

 the whole nervous, circulating, and generative 

 system, the placoids stand at the highest point of 

 a natural ascending scale, and the ganoids arc 

 also very highly organized." Of certain families of 

 the first order, found in the Old Red Sandstone of 

 Russia, he says, " Let the reader bear in mind 

 that these fishes are among the very highest types 

 of their class, and that we can reason upon them 

 with certainty, because some of them belong to 

 families now living in our seas." He instances a 

 cestraceon — a high kind of placoid — recently 

 found in the Wenlock limestone, a low portion of 

 the Upper Silurians, and therefore near the be- 

 ginning of fish. Some of the ganoids, also, of 

 the Old Red Sandstone make an approach to u 

 higher class — reptilia. Besides the usual row of 

 fish-teeth, they have an inner range, in which we 



home by Mr. Murchison are not of the ctenoid order, but belong 

 to a placoidean family called Ctenodus. The mistakes made by 

 this writer, in the geological part of his paper, are of a very 

 grave kind, yet only such as many men of scientific eminence 

 may be expected to make when they venture out of their own 

 peculiar department, and rashly under-estimate the strength of 

 the arguments to which they are opposed. 



I 



