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



centimetres (omitting tlie inferior clavicular expansion), with an inter- 

 space of 2.5 centimetres sej)arating the superior articulating extremi- 

 ties, as compared with' the bone in the second, where the depth is 6.2 

 centimetres, and the interspace above only 1.7 centimetres. 



The bones of this arch are easily taken apart by maceration, and as 

 has already been hinted they resemble each other very closely among 

 the genera. The posterior angle between scapula and cor«coi(Z averages 

 for the species about 60°, and among the Grouse the distal end of the 

 shoulder-blade is usually found to overhang the fourth dorsal i)leura- 

 pophysis on either side ; but in the California Quail these bones actually 

 extend so far back as to overshadow the ilia for nearly a centimetre, 

 they being proportionately narrow for their unusual length. This bone 

 with the Grouse is only Sc5mi-i)neumatic, its pneumaticity being confined 

 to its coracoidal extremity, where the foramen is found in its usual site. 



The blade is thin behind, becoming stouter as it nears the glenoid 

 cavity, sharp along its inner border, rounded without, clubbed at its 

 pelvic end, and turned gently outwards along its entire length. Ante- 

 riorly it contributes about one-third of the surface for the humeral joint, 

 the coracoid the remaining two-thirds, while the acromial process with- 

 in reaches forward barely to touch the clavicle, forming, as it does so, the 

 usual interscapulo-coracoidal canal. 



These bones are narrow in J3owas« and in T. canadensis, more curved in 

 Lagopus, and very stout in Ccntrocercus, a good medium being seen in 

 the Pinnated Grouse (Plate XII, Fig. 85). 



The coracoids are quite stout bones, but devoid of any marked 

 peculiarities; after leaving their sternal beds they extend upwards, 

 forwards, and outwards, the furculum governing their distance apart 

 at their upper ends. The glenoidal process is extensive, and rises 

 nearly as high as the head of the bone; the clavicular process is faintly 

 bifurcated, and curls over slightly towards the fellow of the opposite 

 side. The shafts are compressed from before backwards, and soon 

 dilate into broad expansions below; anteriorly one longitudinal muscular 

 ridge marks the surface of the bone, while on the sternal aspect two or 

 three very distinct ones occur. The inner angle of the dilated base 

 nearly meets its fellow of the opi)osite side through the foramen, at the 

 root of the manubrial process ; from this corner outwards, for about 

 two-thirds of the distance the surface is devoted to a curved articulating 

 facet for the coracoidal groove, while the outer angle is tilted upwards 

 and finished off by a distinctive little tubercle. On its posterior aspect, 

 about the middle of the dilated part, a large, irregular i)neumatic fora- 

 men is found in all the Grouse, but seems to be absent in the Partridges. 



The position of the united clavicles, or the free acromial extensions of 

 the scapula}, is shown for Lagopus in my drawing in Fig. 91. Something- 

 has been said about this bone already ; we will add, however, that the 

 superior ends always terminate by rather tuberous enlargements, smooth 

 internally, but uneven as applied to the clavicular i^rocesses of the 



