OF THE GREAT AUK, OR GARFOWL. 323 



upward, the base intervening between the coracoid (6) and costal (a) surfaces. The 

 latter, occupying 1 inch 9 lines of the sternal border behind the costal process, presents 

 seven articulations (Pis. LI. & Lll. fig. I, a, a) for as many hsemapophyses (ib. h), pro- 

 gressively decreasing in size as these likewise diminish at their sternal ends. The 

 sternum is continued 4 inches 5 lines behind the costal borders, slightly expanding before 

 it is rounded off to the end, which is truncate, and reduced to a breadth of 9 lines. 



The front border of the sternal keel, I inch 10 lines in depth, is concave and carinate 

 (PI. LII. fig. 2), the obtuse anterior angle of the keel being produced toward the 

 furculum (PI. LI. fig. 1, ss), but not reaching or coalescing with that bone. The 

 keel extends, gradually losing depth, to within an inch of the hind end of the sternum, 

 and at its subsidence a pair of curved lines (PI. LII. fig. 1, I), convex backward, 

 diverge to near the lateral borders, and are reflected forward, three or four lines from 

 the border, to the end of the costal surface (a). The total length of the sternum is 

 7 inches 8 lines ; its greatest breadth is 2 inches 3 lines. 



§ 2. Skull. 



The skull is long and narrow, the rostral part forming nearly the two anterior thirds, 

 compressed, and deep ; the orbits are large, with only the upper half of their bony rim 

 defined. The cranium is very small, and chiefly seen at the upper half of the posterior 

 fifth part of the entire skull (PI. LI. fig. 2). The interorbital region slopes to the 

 base of the upper mandible, which, rising at its compressed part, leaves a wide con- 

 cavity in the contour line between the mandible and the cranium proper. Both the 

 temporal (t) and the superorbital glandular (so) depressions are deep and sharply defined, 

 meeting, but separated respectively, at the mid line by a low, sharp crest. 



There is a large lower and a small upper vacuity in the interorbital septum, the former 

 continuous posteriorly with the optic vacuity, the latter with the olfactory vacuity : in 

 the anterior cranial wall there is a pair of vacuities, one on each side of the orbito- 

 sphenoidal base of the ossification, dividing the upper from the lower vacuity in the 

 bony interorbital septum. 



The occipital tubercle is subhemispheroid, projecting below the level of the basi- 

 occipital (ib. fig. 3, i )• This is transversely extended, subcarinate, divided by a pair of 

 transverse curved ridges from the basisphenoid (ib. 5). The paroccipitals (ib. i) are 

 broad, obtuse, trihedral, the narrowest surface being mesiad ; the fore surface is concave ; 

 the outer margin is continued as a ridge upon the occipital surface, defining the share 

 thereto contributed by the mastoid : the fore part of the base forms the posterior tym- 

 panic cup. The mastoid (ib. 8 ) is short, obtuse, inclined downward and forward. The 

 inner and back part of its base is excavated by a deep transverse oval fossa for the 

 anterior condyle of the tympanic element (ib. 28). 



The occipital surface of the cranium is vertical, subsemicircular ; its upper and lateral 

 border forms a deep and sharp ridge, dividing it from the temporal fosssB. Each fossa 



VOL. V. PART IV. 2 u 



