60 
DAVIS : FOSSIL FISH EEMAINS. 
become worn and the fish increased in size, they were replaced by 
a set of larger teeth from the inside of the mouth, somewhat 
similarly to the increase in the sharks, except that the earlier row 
of teeth instead of becoming detached, adhered to the second or 
new row, and becoming firmly cemented to them served to 
strengthen and support them. In this way successive additions 
were repeatedly made, each additional row by its increase in 
laternal extent as well as in length being proportionate to the 
increased growth of the jaw. Specimens have been found with 
five such rows, all firmly cemented and adhering to each other. 
The peculiar constitution of the jaw and teeth of Petalorhyn- 
chus with the median teeth over the symphysis of the jaw is 
aberrant from the usual type of the Selachians, and it has to this 
extent some afiBnity with the Rays. Messrs. Hancock and Atthey 
have pointed out the relationship of Climaxodus, McCo}^, and 
Janassa, Munster, in a paper in the " Natural History transactions 
of Northumberland and Durham, vol. iii., pt. II., p. 330." The 
arrangement of the teeth of those fishes is very similiar to that 
of Petalorhynchus, they extend, however considerably more in a 
horizontal direction, over the palate of the mouth, and in addition 
to the sharp cutting-edge of the extremity of the tooth, the crown 
was developed so as to form crushing or triturating surfaces. 
The authors after a minute description of the specimens arrive at 
the conclusion that Janassa, Munster, approaches somewhat in 
character to Myliobates of the newer formations, which have a 
broad tooth occupying the median portion of the mouth with three 
rows of smaller teeth on each side extending from the extremity of 
the jaws inwards over the palate. Should this relationship be estab- 
lished and confirmed by the discovery of intermediate species, an 
interesting evolutional series may be traced from the Petalodonts 
of the Lower Carboniferous Limestone, through Chinaxodus of the 
upper beds of that series, the Janassa of the Permians, to the 
Myliobates of the newer formations to the members of that genus 
which still exist in the seas of the present time. 
