598 OSTEOLOGY OF LAEID^. 



■with the sternum; the last one does not, although it has a sternal rib attached to it. 

 Neither of them shows traces of posterior oblique processes. 



Sternum. — The stermim varies more in the different subfamilies than any other por- 

 tion of the skeleton except the skull. It will, therefore, be necessary to institute com- 

 parisons between the four types, although a general description may still be made for 

 either of the groups. I describe it held upside down. 



The body of the bone is pretty regularly n-ctangular, and about twice as long as 

 broad across the middle. In Lesiridinai and Slernina; the lateral borders are nearly 

 straight or but slightly concave, while in Larina the outline is very conca^'e from the 

 protrusion of the costal processes. This much greater development of the costal pro- 

 cesses in Larus gives a greater breadth anteriorly, and detracts from the more perfectly 

 rectangular shape which obtains in the other subfamilies. Just posterior to the costal 

 I)rocesses are the six articulating facets for the sternal ribs. These are simply linear 

 perpendicular eminences, separated by curved sulci from eaoli other. They extend 

 bnt little more than half the length of the sternum, the remaining portion of the bor- 

 der being thin and j)hiin. 



The anterior border is very convex, running forward a considerable distance from 

 the costal i)rocesses, having on the median line a large semicircular, well-developed 

 manubrial process. The outer half of this border is thin, corresponding with the costal 

 processes; the inner very thick, and deeply grooved to form the coracoid facets. These 

 facets are of a shape somewhat peculiar, as they curl downward and inward toward 

 each other, to meet on the median line beneath the manubrium, which consequently 

 overhangs them. In the middle of their course they are very shallow, but again, at 

 their outer extremity, become sulci by means of a short, stout process of bone which 

 overhangs them. 



The posterior border differs in the Lcstridino: from the other subfamilies. It has but 

 a single long, slender incurved process, forming with the xiphoid cartilage a single 

 deep oval excavation. In the other subfamilies the outer process is not so long and 

 slender, it projecting but little if any beyond the extremity of the xiphoid; and there 

 is, in addition, a second small, short, slender process midway between the externa] one 

 and the xiphoid, which divides the single membranous space of the Lesiridinw into two 

 small sijaees, each of a more or less oval shape. 



The dorsum of the sternum is strongly vaulted, both transversely and longitud- 

 inally. The amount of convexity both waysis gi'catest in Lcniridhue, least in Sleniiiw. 

 In Lealridina; also, the convexity is very much the greatest anteriorly, while in the 

 other subfamilies the whole dorsum is more equally vaulted. The most noticeable 

 feature of the dorsum is the prominent ridge which extends from the osseous i)rojec- 

 tion at the extremity of the coracoid facets obliquely backward and inward to meet 

 the keel near its posterior extremity. This ridge limits the attachment of tlie pec- 

 toralis medius muscle to the sternum. It is proportionally shortest in the Leslridinw. 



The keel is umarl-.ably well developed, rising high above the dorsum. Its height is 

 equal to half the average width of the Ijone. Its ridge is very convex, and does not 

 lose itself in the dorsum until it has quite reached the end of the bone. Its anterior 

 border curves forward with a strong, wide, and deeply concave margin, being ternji- 

 nated by a well developed and greatly projecting though not very acute apex. 



The xiphoid process is broad, rounded, and flattened in Lculridince, and extends a 

 little way beyond the subsidence of the keel. lu Larince it is narrow and more acute, 

 and the keel is continued to its very tip, while in Sterninw it is so compressed as to ap- 

 pear simply like the termination of the keel, slightly broadened. 



The jmeumatic foramina are very numerous. They are situated along the whole 

 length of the median line of the inner surface; at the base of the anterior border of 

 the keel ; in the sulci between the costal facets, &.c. 



The coracoids are of moderate length and stoutness. The shaft of the bone centrally 

 is snbcyiindrical, bnt at either extremity is greatly expanded in various directions. 

 The basal articulating surface, besides being deeply concave, is twisted upon itself in 

 a peculiar manner to adapt it to the shape of the sternal facet. The inner margin of 

 the bone is thick and rounded, the outer thin and sharp, and expanded into a well 

 maiked rounded i>rocess. The head of the bone presents a large irregular expansion, 

 generally convex in its outer aspect, but made exceedingly thin by a deep concavity 

 or excavation on its inner side. A prominent process at the inner and inferior border 

 (-f the head supports the tip of the furculum. Between this projection and the pro- 

 ce!;S which supports the head of the scapula ia a deep groove, which extends some dis- 

 tance along the shaft. It is nearly bridged over and converted into a canal by an 

 overhanging projection of the scapular process. A dense ligament cimpletes that por- 

 tion of the canal which wants osseous parietes, and through it plays the tendon of the 

 pectoralis medius. 



On the outer aspect of the head of the bone is the depression which contributes to 

 form the glenoid fossa. Neither it, however, nor the scapula itself, present any peculi- 

 arities worthy of special notice. 



The clavicles have, as usual, a scapular as well as a coracoid articulation. The former 



