Pisces. | DEVONIAN VERTEBRATA. 599 
which is bevelled from without with broad squamose sutures, so that it is overlapped by the other four, the two 
anterior lateral of which would be nearly square, but for the truncation of the inner angle by the central plate ; 
the posterior lateral plates are much longer than the anterior, elliptically pointed at the posterior end, each 
marked at about a third from the posterior end by a transverse depression, which is counted as a suture by 
Agassiz and Egerton, the former again bisecting each of the triangles so defined by a second suture, of none 
of which however have I seen any trace, but on the contrary, I think I have seen the parts entire in many 
specimens, as stated by Miller; the outer third of both the anterior and posterior lateral yentral plates is bent 
upwards at right angles, to form a portion of the vertical sides of the carapace. The dorsal aspect has six large 
plates, two median and two lateral on each side. The anterior median plate is obscurely ovato-hexagonal, 
truncated in front, patelliform or obtusely conoidal, and occupying the anteriorly eccentric apex of the back ; 
its anterior lateral margins are bevelled from within so as to overlap the anterior lateral plates, the latero- 
posterior edges are bevelled on the outer edge, so as to be overlapped by the posterior lateral plates ; the narrow 
posterior edge being however bevelled from within, so as to overlap the posterior median plate ; posterior median 
plate ovate, narrow in front, where it is overlaid by the posterior end of the anterior median plate, its long antero- 
lateral edges bevelled from within by a broad squamose suture to overlap the posterior lateral plates; posterior 
lateral plates large, extending not quite so far forward as the middle of the anterior median plate, truncated in 
front, where they join the anterior lateral plates by a transverse suture ; their outer third abruptly bent down- 
wards at right angles to meet the posterior lateral plates in the middle of the vertical sides; anterior lateral 
plates truncated in front and behind, deflected about their middle at right angles to join the anterior latero- 
ventral plates on the vertical sides; their concave anterior margin thickened by a bony internal ridge; a 
similar thick bony ridge is within the concave posterior edge of the posterior dorso-lateral plates, from which 
two internal ridgés run, one nearly to the internal apex of the anterior medio-dorsal plate, the other running 
along the angle of the sides as far as the anterior margin of the plate. The head is covered by several irregular 
polygonal pieces, the exact form of which is still doubtful. The lateral diverging appendages are supposed by 
some to be pectoral fins, and by others to be cephalic spines; their being articulated by the ginglymoid joint 
alluded to, is against the supposition of their being fins; and against both suppositions is apparently the 
existence of a joint at about the middle of their length, where they are widest, and from whence they gradually 
taper towards the body, and are abruptly pointed towards their extremities. The single small fin is supposed 
by Miller to be either dorsal or ventral, and by Sir P. Egerton to be dorsal; I have invariably found it however 
on the ventral aspect ; and as my observations agree with Miller's in the point of its not being paired, I believe 
it to be an anal fin. Sir P. Egerton’s own figure (Geol. Journ. Vol. IV. t. 10) represents this fin on the 
ventral aspect of the tail, for it will be remembered that the dorsal plates there represented are seen from 
within. A ridge of angulated scales extends from this fin to the extremity of the tail, and it is apparently part 
of this ridge originally described by Miller which Sir P. Egerton designates as caudal fins in the above figure, 
no trace of caudal having been seen in other specimens. The restored figure of Pamphractus, in Agassiz’ Old 
Red Fish, t. 6. f. 2, represents the dorsal aspect of a true Prerichthys, the longitudinal suture in the posterior 
median plate being an accidental fracture. The plates are composed of three layers, an under smooth one, a 
middle coarse osseous one, and a very dense, polished, granulated, superficial one, which is easily separated. 
PTERICHTHYS CANCRIFORMIS (A4.) 
Ref —Ag. Old Red, t. 1. fig. 4, 5. 
Desc.—Head prominent, rounded; pectoral appendages narrow, slightly longer than the width of the 
body at their base, terminating in a slender point; length of head and body about three inches, greatest width 
(immediately behind the pectoral appendages) about one and half inches, tapering rapidly from thence to the 
tail, which equals the end of the carapace in width, being little more than one-fourth the width behind the 
pectorals ; tail the length of the pectorals; central ventral plate acute-angled, rhombic, a fourth longer than 
wide, length equalling the width of the anterior lateral plates; plates of the body and pectorals coarsely 
[easc. 1. ] 4H 
