6 Dr. R. H. Traquair—Homosteus and Coccosteus compared. 
The lateral-line system of grooves is sparingly developed on the 
cranial shield of Homosteus. On each side the lateral groove passes 
along the external occipital and the marginal on to the post-orbital, 
where it divides into two branches, one of which passes outwards 
and forwards to the margin of the shield, while the other passes 
backwards and inwards to be lost near the middle of the central 
plate. If we compare this arrangement with that in Coccosteus 
(Fig. 2), we shall see that the plan is quite the same, although the 
extent of the groove-system is considerably diminished. 
The facial bones of Homosteus are extremely difficult of determi- 
nation, and I must frankly confess that I have come to no certain 
conclusions regarding them. In the specimen represented in Pl. I. 
Fig. 1 are three detached bones, a, B, and co, on each side of the 
anterior part of the cranium, by which B and ¢ are also partly con- 
cealed, while on the right side the bone a is seen only in longi- 
tudinal section, having stood on edge to the bedding of the rock. 
When those bones are seen in connection with examples of the 
buckler, they alway occur in the same order, and isolated specimens 
of all of them are also in the collection of the Edinburgh Museum. 
The bone 4 is broadest behind the middle, narrowest at each end, 
especially the anterior one. In the specimen here figured it is seen 
from the internal aspect, having apparently got turned over; but 
other specimens show that on the external aspect near the middle 
it had a patch of the usual tuberculation, with a short lateral-line 
system groove. This bone is figured by Hugh Miller as “ lateral 
cerebral plate,” but as many specimens in the collection show that 
its position was immediately below the edge of the antero-lateral 
portion of the buckler external to the orbit, the groove on its surface 
being a continuation of the transverse branch on the post-orbital, 
it is clearly the homologue of the paddle-shaped bone or mazilla in 
Coccosteus (Fig. 5). If this be the case then, we may assume that 
the bone B, following and parallel to it, is the mandible; but no 
traces of teeth can be found on either, or, indeed, on any bone 
which it is safe to refer to Homosteus. Like the Sturgeon it must 
have been edentulous. 
The bone c is figured by Hugh Miller (Footprinis, fig. 45a) as 
a clavical (here he meant what we now call post-clavicle) ; but, of 
course, such an interpretation founded on its superficial resemblance 
to the post-clavical of a modern Teleostean is here negatived by its 
position. Hugh Miller noticed the tuberculation of the outer side 
of one of its extremities, but in accordance with his theory of its 
position in the animal, he assumed this tuberculated portion to be its 
“head,” or anterior extremity. The present specimen shows, how- 
ever, that this extremity was posterior. 
The last and crowning point of interest in the specimen repre- 
sented in Pl. I. Fig. 1, is the exhibition of the dorsal cuirass in 
situ. Five plates are here seen, one median and two lateral, 
corresponding exactly to plates occupying a similar position in 
Coccosteus. 
The central dorsal plate (m. d.) is so well known as Hugh Miller’s 
