DAVIS : OSTRACACANTHUS DILATATUS. 
193 
point, and are slightly curved ; deep grooves extend quite 
to the point longitudinally along each lateral expan- 
sion. In some respects Byssacanthus presents features 
similar to those of the spine described above. Both are 
obtusely pointed and much expanded towards the base ; but 
whilst in Byssacanthus the anterior portion of the spine is 
round and strong, the posterior is much expanded and ap- 
pears to be thin. In my specimen the basal portion radiates 
equally in all directions from the point. The furrows on 
Byssacanthus also indicate this great difference — they lie 
parallel to the anterior margin from the point to the base 
on the thicker portion, whilst on the posterior wing -like 
expansion they are wider apart and spread rapidly towards 
the base. In my specimen the furrows indicate the homo- 
geneous character of the spine by their similarity in form 
and arrangement on all sides. There is no further indica- 
tion that the spine of Byssacanthus was other than the 
appendage of an ordinary Cestraciont. 
This spine, and its attachment to the dermal covering of 
the fish, bears a strong resemblance to the bony spines of 
the Trunk-fish, Ostracion cornutus. The Trunk-fish is a 
small squarely-built fish, protected by a covering of six-sided 
plates. On its anterior and posterior extremities the dermal 
covering is produced so as to form four bony spines, broad at 
the point of insertion, rapidly contracting, and thence con- 
tinuing to the apex, and ending in a sharp point. The 
spines of the trunk-fish are long and slender in proportion 
to their breadth ; being fully three times as long as the 
diameter of the base. The fossil spine is comparatively 
short, and much stronger, but otherwise they are very simi- 
lar, both in form and method of attachment. 
Hitherto the fishes found in the Coal Measures have been 
classed as members of one of the two great groups which 
formed the vertebrate fauna characteristic of the carboni- 
