314 Proceedings of the Royal Physical Society. 
broken away, while those of the upper margin are well shown. 
In Fig. 3 we have a head in which, though the general form 
and position of the cirri are indicated, their preservation is 
still more imperfect, those attached to the dorsal part of the 
margin having only their extremities exhibited. Again, in 
Fig. 4 we have the anterior part of another head, in which, 
though the dorsal rim of the opening remains, the ventral 
one is broken through in the middle, and only scant traces 
of the cirri remain. 
It is now clear that the specimens formerly described and 
figured by me were imperfect, and that they for the most 
part showed merely the ventral margin of this median 
ring (5, Pl. I, Figs. 2, 4, and 5). - And this explains 
things which, in the specimens then before me, I was quite 
unable to understand—how, for instance, in Fig. 2 of the last 
quoted plate, the apparent anterior margin is slightly convex, 
whereas, had it been the upper margin of a mouth, as I sup- 
posed, it ought to have been concave. On the other hand, in 
the specimen represented in Fig. 1 of the same plate, it is the 
upper margin of the opening which remains, the lower having 
become quite obscured by crushing and weathering; and 
herein is found the explanation of the circumstance that the 
anterior margin of the head in this case projects farther 
forward than in the others. 
What is the nature of this aperture, with its strange fringe 
of cirri? It cannot be a sucker like that of the larval Lepi- 
dosteus, which is a mere superficial disk, having its adhesive 
papillz composed of epithelial cells!) and which has no 
resemblance at all to the part of Palewospondylus under 
discussion, save its position on the front of the head. The 
more obvious comparison—and that which is in harmony 
with the rest of the structure of our fossil—is that with the 
single nasal opening of Myxine or Petromyzon. And if this 
view be the right one, then Palwospondylus is monorhinal, and 
7s a Marsipobranch. - 
Here, however, it must be remembered that both in the 
' Hag and in the Lamprey the nasal canal is continued past 
the olfactory organ, and, passing through the basicranial 
Balfour and Parker, Phil. Trans., 1882, part ii., p. 385. 
