14 
tibial side. Eight lines below the articular surface commences the fibular ridge, which 
has a similar extent; the fibula is anchylosed therewith in the skeleton from the 
mummified specimen, but not in the bones of the Great Auk sent to me by Mr. John 
Hancock, After an interval of separation of about 3 lines, the fibula (ib. 67) coalesces 
in both specimens with the tibia, and can be traced to within an inch of the distal end 
of the bone, The shaft of the tibia soon acquires a form giving an ellipse in transverse 
section, elongated from side to side ; it very gradually diminishes to within an inch of 
the distal end, and then slightly expands to the condyles. he precondyloid groove (f) 
for the tendon of the extensor communis muscle is bridged over by ligament, not by 
bone ; it subsides upon the shaft an inch below its summit; it is submedian in position ; 
its lower outlet is transversely elliptical, and just above the intercondyloid space. Of 
the anterior prominent parts of the distal condyles, the outer (fibular) one (d) is rather 
broader than the inner (a), and is narrower than the intercondyloid space. 
The canal leading to the bridge is wide and bounded by a ridge chiefly on the tibial 
side. The ectocondyloid surface is almost flat, slightly concave; the entocondyloid 
surface is made more concave by the prominence of the periphery of the condyle, and 
is divided by a ridge developing a tubercle towards the posterior part of the condyle. 
The posterior trochlear surface of the condyle is very slightly concave transversely, 
with a low median convexity. The transverse and antero-posterior diameters of the 
distal condyles are equal. 
The metatarse (Pl. I. fig. 1,69; Pl. II. fig. 13), 2 inches 2 lines long, has the 
outer condyloid concavity (6) lower than the inner one (a) anteriorly; the calcaneal 
ridge is low and vertically perforated. In the anterior concavity there is a smaller fore- 
and-aft canal. The inner (tibial) element (a, ii) is the shortest; the trochlea of the 
middle one extends 3 lines beyond it; the cleft between this and the outer trochlea 
extends anteriorly to the lower fore-and-aft canal (c), but not posteriorly. The outer 
condyle (iv) ends about a line above the middle one. 
The inner toe (Pl. I. fig. 1, %), of three phalanges, is 2 inches 5 lines long; the 
middle toe (a2), of four phalanges, is 3 inches 3 lines long; the outer toe (iv), of five 
phalanges, is 3 inches 2 lines in length, and the slenderest of the three, the middle one 
being the thickest. 
Save in parts of the cranium, no bone in the skeleton of Alca impennis is pneumatic ; 
but the humerus has a medullary cavity, as well as the femur and tibia. 
§ 5. Comparison of the Skeleton. 
In Alca impennis there are twenty-two free vertebra between the skull and sacrum, 
in Alca torda twenty-one ; but, in the specimen in the museum of the Royal College of 
Surgeons’ yielding that number, the homologue of the twenty-second vertebra in Alca 
impennis, supporting the penultimate pair of free thoracic ribs, has coalesced with the 
" Catalogue of Osteology, 4to, 1853, vol. i. p. 221, no. 1146. 
