450 CLASSIFICATION OF DEVONIAN FISHES 
zoidal bone, S. O., which presents a short peg-like process in the 
middle of its posterior edge, and has a peculiar raised pattern upon its 
under surface. In front, this bone is articulated with the singular 
four-rayed bone Fr. The posterior ray (with which S. O. is connected) 
is the shortest and broadest of the four, while the lateral rays are the 
longest and the narrowest, the anterior ray holding a middle position 
in this respect. The edges of the anterior and of the lateral rays are 
variously indented, apparently to form an interlocking suture with the 
adjacent bones, while the posterior ray is deeply excavated to unite 
with S.O. A third bone, much smaller than the preceding, succeeds 
Fig. 19. Bones of the head and of the cuirass of Coccosteus. 
them anteriorly, and appears to be separated by a transverse suture 
from a fourth median bone (Pmx.), whose rounded free edge forms the 
anterior contour of the snout. In well-preserved specimens, this edge 
is seen to be beset with small, projecting, spine-like tubercles or teeth. 
The lateral portions of the skull are constituted, proceeding as before, 
from behind forwards, as follows: a triangular bone (Pa. Ep.), one of 
whose sides, directed outwards and backwards, forms the postero-lateral 
side of the hexagon above referred to, unites, by its inner edge, with 
the bone S. O., and, by its anterior edge, partly with Fr. and partly 
with another bone (y). These edges are irregularly sinuous, and form 
a squamous suture with the neighbouring bones. The posterior edge 
