472 DR R. H. TRAQUAIR ON 
remains to settle accurately the relation to those surfaces of the two margins of the 
caudal fin. , 
In fig. 2, Pl. V. of my previous paper, a tail is represented which, though truncated 
behind, shows clearly that on one margin the fuleral scales are much larger than on the 
other; and an additional difference is, that those of the smaller series are, in front, 
peculiarly short and erect. Then, an inspection of fig. 1, Pl Il. of the present 
Supplement shows that the caudal fin, though not bilobate, is unsymmetrical above 
and below, and projects further back on that aspect on which the larger fulcra are 
placed ; moreover, we observe that the lateral scales which clothe the fin are, under the 
line of larger fulcra, also larger. In other words we have, to all appearance, a normal 
piscine heterocercal tail, of which the longer margin, provided with the larger fulcra, is 
presumably the dorsal one, and it now remains to prove with which aspect of the 
carapace this margin of the tail coincides. This question, I maintain, was already settled 
by the specimen represented in Pl. V. fig. 1 of my previous memoir, in which the line 
of smaller fulcra is traceable to the apparent cloacal opening at the posterior extremity 
of the median fold of that plate, which is certainly the median plate of the oral aspect 
of the carapace. 
But a still more complete demonstration of these relations is afforded in Pl. ILI. of 
the present Supplement. Here we have a specimen seen from the oral side, as shown 
by the form of median plate (.v.) with its posterior fold, the presence of the sensory 
plate x. with its orbital (?) perforation, and of the plates a.v.l. and p.v.l., designated by 
me anterior and posterior ventro-lateral respectively. It may also be noted that the 
median plate (m.d.) of the aboral side, seen from the internal surface, is shown displaced, 
and projecting from below the root of the tail. A considerable part of the caudal fin 
with the fulera on both margins is shown, and, with absolute clearness, the line of 
smaller fulcra (v,f:) is seen to proceed forwards and end at the posterior extremity of 
the great median plate of the oral surface of the carapace. Compare this figure with 
the two in Pl. V. of my former memoir. 
It is therefore proved, beyond all possibility of doubt, that that margin of the 
caudal fin which carries the row of smaller fulcra is coincident with the oral aspect of 
the carapace ; and conversely, that the other margin, which projects further back, carries 
the large fulcra, and presumably contained the caudal body-prolongation, is coincident 
with the aboral one. If, then, the tail is constructed according to the normal piscine 
heterocercal type, the aboral surface of the carapace is the dorsal, and the oral one ts 
the ventral surface. 
But it may be asked whether in Drepanaspis the heterocercy might not have 
been reversed as in the reptilian Ichthyosaurus, by the caudal prolongation of the 
body axis having passed down along the ventral margin of the caudal fin, instead of 
along the dorsal one. In that case I should still be in the wrong as regards the 
orientation of the two surfaces of the carapace ! 
In the first place, we know of no such case among fishes. For, though the lower 
