446 Prof. E. W. Claypole—Upper Devonian Fishes of Ohio. 
view this shark shows the ordinary inferior mouth and sharp snout 
of the family. But its head is extended laterally into a thin, sharp- 
edged flange, on the margin of which are set the eyes. This flange 
suddenly terminates backwards, its edge turning in at right angles 
to the axis of the body. Its dorsal aspect consequently presents a 
head twice as wide as the body and semicircular in outline at the 
front. Such a form may without much effort of the imagination be 
compared with that of the Cladodonts (see Fig. 1). 
Monocladodus Clarkt, Qaypota 
63” 
Fig. 2.—Head, pectoral fins, and abdominal region of Monocladodus 
Clarki, Claypole; with teeth enlarged. 
A yet more singular expansion of the head occurs in the Hammer 
Shark. But the great lateral extent in this fish is less suggestive of 
the head of these Cladodonts than is that of the Bonnet Shark. 
It is not my intention here to imply that any close affinity existed 
between this fish and the ancient Cladodonts. This may or may not 
have been the case. Animals far asunder in our system frequently 
simulate one another’s form, so that no argument can be drawn from 
mere external similarity. The condition of these pyritized fossils 
prevents for the present any detailed study of their structure. 
In one or two species there is a remarkable expansion of the skin 
or some membrane asa horizontal flap just in front of the caudal fin. 
I know no analogy that exactly meets this structure. Nor can I 
assign a use for this organ. In one or two specimens it is exceed- 
ingly well-preserved. In other species it may have been absent, or is 
concealed or destroyed. The tail and adjacent parts are frequently 
in bad condition. The flap shows no rays or markings of any kind. 
It was not covered with shagreen and was evidently quite soft. It 
