638 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 22, 
of the narrow posterior extension of the fin, though the marginal 
hour-glass-shaped plates are regularly disposed immediately within. 
the posterior slope. Nothing is to be seen of the anal fin, except the: 
base of the anterior portion, at which point the hour-glass-shaped 
plates are distinctly displayed ; and traces of them are visible almost: 
to the tail. The remains of both ventral fins (d) are traceable 
immediately in front of the ventral plates, over the anterior of which 
one of them is spread, the rays being directed backwards. 
The tail is admirably shown, and is exceedingly perfect. The lobes 
are expanded, exhibiting the depth of the bifurcation. The mar- 
ginal fulera, plates or scales (f), and the double row of rhombiform 
scales (¢) behind, as well as the fin-rays of both divisions, are very 
perfect, the latter displaying their bifurcated extremities stretched 
out. The lobes are about equal in length, if measured from the root, 
along the axis. In this way they are 11 inch in length; but if 
measured along the posterior margin, the upper lobe is nearly 14 inch 
long, and the under lobe only < of an inch in length. The width of 
the tail from tip to tip is upwards of 12inch. The specimen measures 
43 inches in length, and 14 inch in depth. This specimen remains 
in the possession of Mr. Duff, to whom we are indebted for the dis- 
_ covery of these and many other interesting fossil remains. 
Our third specimen (Pl. XLIII. fig. 3) is the only one which hes 
with its right side exposed; all the other three have the left side 
uppermost. This measures in length, including the tail and making 
allowance for what is wanting in front, 43 inches, and in depth at 
the widest part 2% inches; and it is chiefly distinguished by the 
large portion that remains of the dorsal fin. 
The head does not appear to have .been much injured when the 
specimen was deposited ; but unfortunately the upper and frontal 
portions are broken away. The orbit ‘w), however, is well marked ; 
and so are the boundaries of the opercula (s), the posterior contours of 
which are quite distinct. Three or four of the ventral plates (m) are 
seen in their proper places in front of the anal fin, sending upwards 
their columns, which exhibit well their compound and twisted ap- 
pearance. Some of them reach the lateral plates near the occiput. 
The great abdominal rods (0) are seen in connexion with the poste- 
rior plate inclining upwards and backwards. 
The plates and rods are much displaced ; but, notwithstanding, 
their connexions and characters are in some places well seen, 
particularly the relationship of the lateral plates to the lozenge- 
shaped central plates, or areas, on a few of which the median ridge 
is quite distinct, and the continuity of the dark enamel-like surface- 
matter with that covering the lateral plates is very obvious. 
Considerably more of the dorsal fin (a) is present than in any of 
the other specimens. One inch and three-quarters of it, in a good 
state of preservation, inclines backwards; and in front, arranged in 
exact order, are twelve or thirteen short plates, the fin-fulera(a’); but 
these so imperceptibly graduate into the regular rays that it is diffi- 
cult to determine the exact number; they are strong, pointed, and 
have the root distinctly rounded. The broken extremity of the fin 
