PROFESSOR OWEN ON THE GENUS DINORNIS. 361 
poral bone” reduced, in human anatomy, mainly to the support of the tympanic mem- 
brane. The feebly marked groove and ridge on the hind concavity of the tympanic 
intimates the degree in which the function in relation to the organ of hearing is 
exercised in the ornithic modification of the air-breathing vertebrate tympanic bone. 
The vertical length of the present tympanic is 1 inch 2} lines; the long diameter of the 
distal articular surface is 1 inch; it is slightly convex, but by a downward production of 
the middle of the outer border is there made concave transversely at the outer half. 
The mandible of Aptornis presents in profile (Pl. XL. fig. 1, 29-82) a series of graceful 
curves. By the downward production of the angle (30) and the elevation of the articular 
surface (29), it is curved for a short way with the concavity below, then becomes con- 
cave above for a longer extent, and finally is again bent with the concavity downward. 
The extent of the symphysis (Pl. XLI. figs. 6, 7, 32) is 2 inches, following the curve, 
rather more than one-fourth the length of the mandible; its upper surface (ib. fig. 6, 32) 
is longitudinally convex, transversely concave, deepening, as it widens, backward. The 
hinder half of this concavity is smooth; the fore half shows two parallel longitudinal 
nervo-vascular-tracts, with canals leading forward and opening upon the outside of the 
symphysis near the end; this has a horizontal, subtrenchant, slightly convex border. 
The dentary margins are rather sharp as they extend backward for an inch and a half, 
then begin to thicken into a convex border, 1} line across, which border again contracts 
before it is lost in the thick hinder half of the ramus. The outside of the trenchant 
margin of the symphysis is indented by a delicate line. An oblique groove, beginning 
about the middle of the outside of the ramus, indicates the junction of the dentary 
(Pl. XL. fig. 1, 52*) with the angular element (20), which is further denoted by a ridge 
continued backward from the groove. A fissure on the upper border of the ramus, half 
an inch in front of the articular surface (Pl. XLI. fig. 6), sinks into the substance of the 
bone as it advances, and opens by a shorter fissure upon the inner surface of the ramus 
nearly two inches from the angle. The tract between the two fissures probably included 
the hind end of the splenial. Save at these indications all the elements of the mandible 
are fused into one bone of a strong, compact osseous tissue, as in a few other strong-billed 
birds’. The articular surface is single (Pl. XLI. fig. 6, 29) with the moderate convexities 
and concavity of its undulated surface answering to those on the tympanic. ‘The fore- 
and-aft diameter of this surface of the mandible is 7 lines, the transverse diameter is 
7} lines. I never saw a mandibular condyle of an oviparous Vertebrate in which so 
large a proportion was convex. Behind the articular surface there is a very small and 
not deep fossa; internal thereto the bone extends obliquely upward and inward into an 
irregular conical process, attached by ligament to the posterior non-articular part of the 
mandibular end of the tympanic. The outside of the ramus, below the articular surface, 
is strengthened by a tuberous ridge, between which and the angle is a broad oblique 
channel. ‘There is no “coronoid” elevation; the ramus rather loses than gains 
* Ramphastos, Buceros, Psittacide, Balceniceps. 
VOL. VIL—PaRTV. January, 1871. 3D 
