318 PROFESSOR OWEN ON THE SKELETON 
impennis, I wrote for the loan of those of the left side, and was favoured by a prompt 
and kind acquiescence, the bones being stated to be from a mature female bird. 
T have thus at command the materials for a description of the complete osteology of 
this most rare and now generally regarded as extinct bird. 
In my ‘ Descriptive Catalogue of the Osteological Series contained in the Museum of 
the Royal College of Surgeons of England,’’ I have briefly noticed characteristics of the 
cranium, dorsal vertebre, scapula, coracoid, femur, and tibia of an Alca impennis which 
the founder of the collection, John Hunter, had succeeded in procuring; Mr. Blyth 
had previously made known the fact that the humerus “ possessed a very small internal 
cavity, while the tibia was completely filled with marrow’’*; and these are the only 
published notices of the osteology of the bird with which I am acquainted. 
§ 1. Vertebral column. 
In the present specimen there are twenty-two moveable vertebre between the skull 
and sacrum, the last nine supporting moveable ribs, of which the first two pairs have 
free extremities ; the succeeding pairs of free pleurapophyses articulate with hemapo- 
physes, and these with the sternum. 
The sacrum appears to include fourteen vertebra, of which the first supports a tenth 
moveable pair of ribs, the last of the ordinary thoracic costal series: its hemapophysis 
does not reach the sternum. 
The caudal vertebrz are fourteen in number, of which the last three are blended 
together, and the first, by its pelvic relations, might claim to belong to the sacral 
series. 
The centrum of the atlas, anchylosed as an odontoid process to the axis vertebra, 
presents a pair of small facets for articulation with the posterior basal angles of its 
proper neurapophyses ; but these are mainly supported by the hypapophysis, simulating 
the body of the atlas, and with which they are confluent. The back part of the hypa- 
pophysis offers a flat surface to the centrum of the axis, beneath which it is slightly 
produced at its lower part, being here wedged into the notch between the true bodies of 
the axis and atlas. The fore part of the hypapophysis combines with the neurapophyses 
to form the cup for the condyle of the occiput; the cup is emarginate above, and 
traversed by the ligamentous continuation of the ‘‘ odontoid”’ in its way to adhere to the 
upper part of the occipital ball. The atlantal neurapophyses diverge as they rise, and 
are joined together above by a broad plate slightly arching across from one to the other 
neurapophysis. A process extends backward from each place of junction. There is 
no neural spine. The neural canal has a wide transversely elliptical area. 
The body of the axis is elongate, compressed and carinate below, the keel being 
slightly produced and curved at the hind part. The expanded anterior end of the 
centrum has a small notch near or at the junction of the neurapophyses. These deve- 
* 4to, 1853, vol. i. p. 221, preps. nos. 1150-1160. ? Proc. Zool, Soc. November 14, 1837, p. 122. 
