(JLASSIFICATION OF DEVONIAN FISHES. 



31 



In the middle Hue, behind, they mark off a symmetrical, trape- 

 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 sin- 

 gular 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 them ante- 

 riorh^, 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 of Pa. Ep. presents, 

 near its inner extremity, a sort of socket, with which a peg 

 developed from the plate S.s. is articulated. 



An irregularly triangular bone {y) is connected with the anterior 

 edge of Pa. Ep., and forms the posterior angle and part of the 

 antero-lateraledgeof the skull. It is succeeded by another irregular 

 bone (x), which enters into the posterior and upper wall of the 

 orbit, and unites internally with Fr., and anteriorly with a larger 

 and still more irregular bone Pfr. The latter is connected in- 

 ternally with Fr., Eth. and Pmx. ; while externally it sends off, 

 rather in front of the middle of its length, a short process, which 

 passes directly downwards and divides the orbital cavity (Orb.) from 

 the nasal cavity (Na.) Extending from the lower end of this process 

 to the postero-lateral angle of the skull, bounding the orbit below, 

 and fitting in by its convex margin, to a concave excavation of the 

 bones x, y, is a large spatulate bony plate, narrow in front, broad 

 and expanded behind, and composed of two pieces, z and z\ 



The cranio-facial shield thus composed is succeeded by an osseous 

 girdle, which defended the anterior part of the body like the back 

 [x.] 10 c 



