148 SHOULDER-GIRDLE AND BREAST-BONE. 



and it is not thoroughly ossified. Opposite the sixth sternal rib there is, in the specimen in 

 ray collection, on the left side, an oval " fenestra," an arrested cleft between the ento-sternum and 

 the costal region ; and all along this region there are large patches of bone deprived of diploe, 

 showing a tendency to split up into parallel bands. 



In Uria grylle (see Owen, 'Trans. Zool. Soc.,' 1865, vol. v, pi. 52, fig. 14) the Sternum is 

 much wider in the external xiphoid region ; and there are two small fenestrae one space divided 

 by a periosteal band on each side, showing the boundary line between the "intermediate" and 

 " middle xiphoid" regions. In Alca impennis these spaces are obsolete (see fig. 1 of the same 

 plate), and in Alca Torda (in my specimen) the short ensiform "external xiphoids" are separated 

 by a notch from the middle part, whilst on the right side there is a small fenestra between the 

 intermediate and inner regions. In the specimen in the Hunterian Museum (No. 1146) there 

 is no sub-mesial fenestra, and the outer notches are walled-in, having either become fenestrse 

 in old age, or having been so from the first. In Fratercula arctica (No. 1161) the outer 

 clefts are merely two minute foramina on the left, and one small hole on the right side ; the 

 inner spaces are much larger, and are fenestrte ; behind these arrested clefts the Sternum is 

 narrower, but then sends out two small ears, and terminates in a large semi-oval spoon-like plate. 



The Colymbidae (see Hunt. Mus., No. 1165) differ much from the Alcidae, for the scapula is 

 relatively much shorter, and the coracoids altogether larger ; the meso-scapula is blunter and 

 smaller, and the meso-coracoid is a delicate curled flap of bone, separated by a deep notch above 

 from the main part. The " prae-coracoid segment" is not developed, and the furcular rami are 

 very flat and thin ; their principal curve is at the lower third ; the inter-clavicle is small. The 

 rostrum of the Sternum is much less pronounced than in the Alcidae, and the keel is less developed 

 at its angle ; it also dies out sooner behind, and is less deep. The whole Sternum is broader and 

 much more scooped ; the external xiphoids are narrow, being separated by a deep notch from the 

 main part ; this has no very distinct sub-mesial fenestra, but in C. septentrionalis the broad, flat, 

 abdominal part is notched feebly on each side, and is full of small holes ; this is like what is 

 seen in the coracoid of the Herring (see Plate II, figs. 4, 5), where the cleavage is broken up into 

 many spaces. 



Family PODICIPIN^E. 



Example 1. Podiceps minor, Gmel. 



These Birds agree in many respects with the Colymbi, but they are not unrelated to the Cormo- 

 rants. The Sternum of the small kind, the Dabchick, is shown of the natural size in Plate XIV, 

 figs. 4, 5, as seen from above, and laterally. There is no rostrum, and the keel (k.) does not 

 project much forwards ; the coracoid grooves (cr. g.) are wide apart, and a rounded (primordial) 

 notch separates them ; behind, there is a larger amount of this original divisional space. The costal 

 processes are triangular ; the costal condyles are six in number on each side ; the external xiphoids 

 are long and elegantly bowed ; they are separated by a large semi-oval notch from the interme- 

 diate xiphoid, which, in its turn, is but feebly marked off from the short and somewhat arrested 

 ento-sternum. There is generally some unossified cartilage behind, and the bone is extremely 

 thin at that part ; on the whole, this is one of the most arrested types of Sternum to be seen in 

 the whole Class, and differs very widely from that of the Colymbidae and Alcidse. The coracoids 



