THE GEOLOGIST. 



MAY 18G3. 



CESTRACIONT FISHES OF THE CHALK. 

 Br tile Editok. 

 TnE very fine specimen of palatal tooth of Piyclwdus pohjt/yrus, 

 which we figure in Plate IX., from the collection of N. T. "YVetherell, 

 Esq., of Highgate, temporarily draws our attention to a class of re- 

 mains of very considerable interest. 



We have not the leisure at the present time for going as deeply 

 into the subject as it well deserves, nor as the mass of valuable ma- 

 terials accumulated since the publications of Agassiz in 1813, and 

 Dixon in 1850, require. 



There are also other important points than the mere bearings of 

 more detailed information of the characters of species very possibly 

 to be gained by a study of the singular and marked group of cestra- 

 ciont fishes. First known, in abundance of* individuals, in the Car- 

 boniferous age — though not at any time numerous in genera, — and 

 presenting various forms, numerically abundant, in the J urassic and 

 other intermediate formations up to the Chalk, characterized by its 

 many varieties of Ftychodus, but now dwindled down to a Bolitary 

 representative in the Port Jackson shark, it is one of those very 

 circumscribed groups in which we ought to find more especially and 

 distinctly marked traces of the transmutation of one species into an- 

 other, if such transmutation did exist in the pasl ages of our planet. 

 That the group does present important evidence on this point is cer- 

 tain, but whether sufficient or not to come to a practical and definite 

 conclusion, may be as yet doubtful ; although, if collectors will turn to 

 the fossil remains of these fishes in earnest, we may rest assured of 



VOL. VI. Y 



