330 PROFESSOR OWEN ON THE SKELETON 



the distal end, and then slightly expands to the condyles. The precondyloid groove (/) 

 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 (6) 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 (PI. LI. fig. 1, 69; PI. LIT. 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 (PI. LI. fig. 1, ii), of three phalanges, is 2 inches 5 lines long; the 

 middle toe {Hi), 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 vertebrae between the skull and sacrum, 

 in Alca tarda 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 

 sacrum. The last pair of ribs, articulated in both species to a sacral vertebra, have haema- 

 pophyses which do not reach the sternum. The chief numerical difference in the vertebral 

 column is in the excess of caudal vertebrae in the Garfowl. The dorsal vertebra; have 

 the same simple shallow ball-and-socket joint, the ball being anterior in Alca tarda, as in 

 A. impennis ; and this structure I have also found more feebly developed in the dorsal of 

 Uria grylle : in Phalcris both co-adapted surfaces are nearly flat. The main difference 

 in the skull is the shortness, greater relative depth, and stronger convex curve of the 

 ' Catalogue of Osteology, 4to, 1853, vol. i. p. 221, no. 1 146. 



