328 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Vol.YI. 



In the Tetraonidcc, following the general aphorism of the class, we ob- 

 serve a gradual dilation of the skeleton from the Inter-roracoidal aper- 

 ture towards the pelvic extremity of the body. To comi)ensate, or rather 

 what really gives rise to this, as far as the bird's osseous structure is 

 concerned, among other things, is a lengthening of the diapophyses as 

 we proceed in that direction. The ribs also become longer ; the xi])hoidal 

 forks of the sternum flange outwards. So, too, we find a corresponding 

 shrinking of the tubercula on the dorso-vertebral i^leurapophyses, and 

 a stretching of the correlative necks, so that the capitula may reach the 

 facet intended for them on the vertebrre. 



The vertebral ribs as seen in the Grouse are x)neumatic, the foraraina 

 being found on the posterior as])ect of these bones immediately below 

 the tubercles, sheltered by the flattened bodies as they dilate in that 

 region. 



As in nearly all birds, these ribs are compressed from within outwards, 

 and in Centroccrcus are wider below the uncinate processes, contract- 

 ing as they meet their sternal ribs. In these birds, too, the epi-pleural 

 appendages are firmly knit to the body of the rib — though in Bonasa,La- 

 gopus, and Tetrao they may be easily detached, leaving behind them in 

 each case a shallow concave facet. In the last dorsal rib this append- 

 age is the smallest, and is directed upwards nearly parallel with the body 

 of the rib, and sometimes {Centrocermis) the edge of its rib swells out to 

 meet its apex above; and this may have been the method by which the 

 unique and remarkably striking uncinate appendages were formed in 

 Ciqndoma, a bird that constitutes itself an exception among the IS'orth 

 American Grouse in this respect (PI. XI, Figs. 79, 80), where not only 

 the vertebral ribs are unusually broad, but their processes much more 

 so, being great, odd, quadrate and compressed plates confluent with the 

 body of the rib, and only rarely condescending to be weakened by the 

 merest apology for a foramen to exist in them, as in Fig. 80. The Sharp- 

 tailed Grouse appears to come next in approaching this state of affairs ; 

 it, too, invariably develo])ing more or less expanded ribs. The only 

 sacral pair of pleurapophyses conform to the general shape of the series 

 to which they belong, and never supj)ort epi-pleural processes. 



Tbese bonj- protectors of the thoracic parietes were found to be 

 exceedingly delicate in specimens of Bonasa kindly sent me by Leslie 

 A. Lee, Esq., of Brunswick, Me. 



Four hfcraal ribs ascend from the costal borders of the sternum to 

 meet the dorsal pleurapophyses in arthrodial articulations above the 

 mid-horizontal plane of the bird's body. As is usual, their sternal ends 

 are twisted so that the facets are situated transversely, while the upper 

 extremities are broad and flat, particularly in dqiidonia and Pediwcetes. 



The first pair are generally in advance of the others, and insert them- 

 selves higher up on the costal processes : The pneumatic foramina for 

 these bones are to be found just above their tranverse facets anteriorly. 

 The extremities of the hiTemax)ophyses of the sacral ribs meet the pos- 



