THE GEOLOGIST. 
MAY 1863. 
CESTHACIONT FISHES OF THE CHALK. 
By the Editob. 
The very fine specimen of palatal tooth of Ftycliodus polygyrus, 
which we figure in Plate IX., from the collection of N. T. AV^etherell, 
Esq., of Highgate, temporarily draws our attention to a class of re- 
mains of very considerable interest. 
AVe 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 1843, 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 Jurassic and 
other intermediate formations up to the Chalk, characterized by its 
many varieties of Ptychodus, but now dwindled down to a solitary 
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 past ages of our planet. 
That the group does present important evidence on this point is cer- 
tain, but whether sulficient 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. X 
