ree 
Cope.] 576 {March 7, 
curved rib-like bones lie parallel and divergent posteriorly on the right 
side of the frontal, in the temporal fossa. I cannot identify them. They 
are not present on the opposite side. As already described, there is a 
facet on the infero-posterior face of the postfrontal process. “This in- 
dicates the point of articulation of the palatopterygoid arch, as it exists 
in the group Opistharthri of the sharks as defined by Gill, and as is clear- 
ly proven by the specimen now to be described. 
This includes the entire palatopterygoid and mandibular arches of one 
side, and the greater part of that of the opposite side, together with a 
considerable part of the right hyomandibular bone and probable ex- 
tremity of the ceratohyal. The anterior parts of both jaws support 
numerous small teeth, which closely resemble those described by Agassiz 
as belonging to his D. gibbosus. They differ from those of the D. compres- 
sus in their smaller size. The palatine bones do not project much beyond 
the mandible, which, taken in connection with the form of the muzzle 
above described, renders it probable that the mouth was nearly terminal. 
In the palatopterygoid arch there is no noticeable separation or suture 
between the palatine and pterygoid elements. The inferior border of the 
palatine is swollen below the orbit; its superior plate rises into a strong 
suborbital ala, which is concave externally, with thin superior edge. This 
edge rises posteriorly, giving the outline an elevated convexity, whose 
greatest upward prominence is above a point a little posterior to the 
middle of the jaw, and which probably articulated with the postorbital 
process of the cranium. Its surface gives indication of an articular sur- 
face appropriate to the corresponding one of the cranium. The superior 
border then descends rapidly to a vertical posterior border, which forms 
a somewhat prominent rim. This descends to the mandible, forming a 
regular ginglymus, the mandible bearing the cotylus. The mandible is 
rather robust; its inferior edge is rather thin, and becomes incurved 
anteriorly. Its superior border is regular, except that it rises a little at 
the coronoid region, and is impressed, corresponding with a concavity of 
the surface, and arch of the border of the pterygoid region, just anterior 
to the posterior prominent ridge which forms its posterior edge. 
The hyomandibular bone is only exposed for its inferior half. It issues 
from behind the palatopterygoid as a narrow shaft with obliquely truncate 
extremity. 
It is thus evident that the arrangement of the jaws is as in the two ex- 
ceptional existing genera, Hexanchus and Heptanchus. 
The external nostril already referred to, is a distinct, rather small fossa, 
on the lateral part of the superior face of the muzzle, near the extremity 
of the osseous portion. It is visible on both sides of the best-preserved 
specimen. It is continued forwards as a shallow groove. At the apex of 
the muzzle, is a fossa looking downwards, where roofed on each side by 
the ? nasal bones, which may represent the posterior nasal cavity. Or the 
latter may probably be represented by a lateral fossa just in front of the pre- 
orbital process. In either case it is evident that the nares are separated, 
