BRITISH ASSOCIATION. 



261 



bral column resembling that of the sturgeon, a character 

 common to the greater part of the species found in those 

 ancient rocks. He remarked the great difficulty of compar- 

 ing these forms to any recent types, the resemblances being 

 only partial, and confined to particular parts of their struc- 

 ture. Thus the helmed heads of the sturgeons and the gra- 

 nulations which protect the heads of Trigla, Dactylopterus, 

 &C.5 resemble a little the Cephalaspis and Coccosteus. The 

 appendages of Pterychthys might, perhaps, be compared to 

 the moveable suborbitaries of the Acanthopsis, or with the 

 elongation of the preoperculum of certain species of Trigla 

 and Cephalacanthus. He pointed out the analogy between 

 the imperfect developement of the vertebral column, and the 

 interior position of the mouth in these genera, with the 

 form of the dorsal chord and position of the mouth in the 

 embryo of fishes. 



3. A third group of the fish of this formation, were cha- 

 racterized by the structure of the ventral fins, which in the 

 genera Dipterus, Osteolepis, Diplopterus, and Glyptolepis, 

 were double, resembling the caudal fin in appearance ; these 

 genera differ from another in the structure of their teeth. 



4. A fourth group were distinguished by their large coni- 

 cal teeth placed in the side of the jaws alternately with 

 smaller teeth. This structure obtains in the genera Holop- 

 tychius, Platygnathus, and Mr. Owen^s genus Dendrodus. 

 This original diversity of types in the fish of so ancient a 

 formation, M. Agassiz regarded as a great argument against 

 the theory of the successive transformation of species, and 

 the descent of all living organized beings from a small num- 

 ber of primitive forms. The result of all his observations 

 concurred in proving the appearance of new species with each 

 formation. 



Mr. Murchison exhibited the drawings of fish belonging to the ge- 



T 2 



