192 
DAVIS : OSTRACACANTHUS DILATATUS. 
fish. The principal part of the fossil consists of a 
bony protrusion, or spine, 1*4 inches in length at the longest 
side preserved. Its breadth at the base is *5 of an inch. 
A portion is broken away; when perfect it would probably 
be *7 of an inch in diameter. From the base, the diameter 
diminishes rapidly, and at half an inch from the apex it is 
•15 of an inch. This diameter is maintained for *4 of an inch, 
and the spine terminates in an obtuse point. Originally 
circular, the base, more especially, is now compressed to 
an oval form. The upper part of the spine is smooth 
and covered with hard ganoine. The lower part is grooved. 
The grooves are longitudinal, and increase rapidly in 
number towards the base by bifurcation. The spine 
appears to be solid; no internal cavity or canal is dis- 
tinguishable in this specimen. The base of the spine is 
composed of chondroid bone : i.e., cartilage with numerous 
osseous centres, but not completely ossified, a similar 
structure to the bony parts of Pleuracanthus (Diplodus). 
Extending from the base, there is a mass of similar bony 
matter. Contiguous to the spine this is produced into two 
or three short denticles. It then becomes thinner, but again 
developes into a mass which may very well have served as 
the base of a second spine, if one were present. 
There is no evidence, Jiowever, of a second spine remaining 
on the present specimen ; it has rather the appearance of a 
thick scale, somewhat accuminate towards the centre. 
Prof. Agassiz, in the " Pois. Foss. des Vieux Gres Kouge,' 7 
describes the genus Byssacanthus with three species, viz., 
B. crenulatus, B. Icevis, and B. arcuatus. The two former 
are figured (Table 33, figs. 11 to 14, and fig. 15). The genus 
is defined as containing spines more or less arched, longitu- 
dinally furrowed, with the base much expanded. The spines 
are about an inch in Jength and three-quarters of an inch 
wide at the base. They converge rapidly to an obtuse 
