330 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Vol.Yl. 



aware that it represents tbe confluent lipemal spines of the free thoracic 

 ribs, not developed in the orderof a spine for a rib, comjiletiui^ the in- 

 verted hiTeiual arches separately, as in some vertebrates, which by their 

 peculiar skeletal toi^ography such niceties become feasible, but, as we 

 have seen, and the fact is not new, by a fewer number of ossific centres, of 

 necessity, due largely to idiosyncrasy of conformation as subservient to 

 purpose, and a convergence of the inferior extremities of the several 

 hpemapophyses to which, as a whole, it belongs. 



In Plate VI, Figs. 52, 51, and 55 are all parts of the skeleton of the 

 same bird — an old adult Sage Cock, Gentrocercm — of which Fig. 54 is a 

 view from below of the sternum. 



It will be seen that it has a length of 14 centimetres, and other meas- 

 urements can be easily obtained from it. We have never seen this bone 

 any larger, and, as it is, it represents the maximum size the hsemal 

 spine attains among North American Grouse. The bone is shown in other 

 plates also, and their owners can be ascertained if the reader will kindly 

 turn to the "Explanation of Plates" accompanying this paper. 



The unique outline of the sternum of the Gallincc has long been known, 

 many authors having both figured and described it, and we will say here 

 that in the Tetraonidcc of our country no marked deviations are to be 

 noted from the more common type. 



Anteriorly the manubrium juts out as a quadrate process with 

 rounded angles ; its inferior margin is continuous with a line that runs 

 down between the slightly i^romiuent carinal ridges, to become continu- 

 ous below with the anterior carinal margin. 



Above, the general surface of the sternal body extends over it. A 

 subcircular foramen, connecting the coracoidal grooves, pierces it at 

 its- base. The coracoidal furrow, thus becoming one groove, is biconvex, 

 being depressed mesiad behind the manubrium, in which depression 

 another pneumatic foramen usually occurs. Their upper and lower 

 margins are produced slightly outwards, the inferior being the sharper 

 of the two. 



The '' costal processes " are exceedingly prominent, being bent over 

 anteriorly at their apices, which are rounded. Behind them are observed 

 the limited " costal borders," exhibiting the four transverse facets for 

 the sternal ribs and i^neumatic foramina. The " carina" or keel affords 

 the greati'st amount of osseous surface of the entire bone, greatly ex- 

 ceeding the body. Its lower margin is a long convex curve outwards, 

 and the " carinal angle " protrudes forwards nearly as far as the manu- 

 brium, causing the anterior margin of the keel to be markedly concave. 



The "carinal ridge" is thickened and heavy superiorly, where it 

 limits or rather constitutes the boundary of the bone in that direction. 

 Below it spreads out, and is gradually lost, icithin the boundary of the 

 carinal border proper. 



" Subcostal" and " pectoral ridges" are nearly always well defined. 



The superior and inferior xiphoidal processes are very characteristic 



