202 NEW YORK STATE MUSEUM 



of the postacetabular portion of the sacrum. External to these 

 gaping sacroiliac sutures, the ilia themselves are broad and flat, and 

 slope away upon either side. Laterally, their rounded borders be- 

 come thickened, and are remarkable from the fact that either one of 

 them far overhangs the side of the pelvis, even extending out- 

 ward very considerably beyond the slender postpubic element. 

 This latter is short behind, and extends but very little beyond the 

 ischium in that locality. There are 16 vertebrae firmly fused to- 

 gether in the sacrum of Tympanuchus americanus. 

 Altogether, the pelvis of this species is quite unique for a grouse, 

 and very different indeed from anything we have among our 

 tetraonine types, with perhaps the exception of Pediocaetes, the 

 species of which genus seem to approach the Prairie or Heath hens 

 in the pattern of their pelves, as they do less in some other partic- 

 ulars. 



Tympanuchus americanus has still another remark- 

 able skeletal peculiarity. The bodies of its ribs are very broad and 

 flat, being laterally compressed, and in the same plane with the 

 large, quadrate epipleural appendages. These broadened ribs with 

 their large processes are best developed in the mid dorsal series, 

 as we would naturally expect. And the superior ends of the costal 

 ribs that belong to, or rather articulate with, these vertebral ribs are 

 likewise broadened, and these characters of the ribs are often found 

 in skeletons of Pediocaetes phasianellus Colum- 

 bia n u s , but not nearly so broad as they are in Tympanuchus. 

 Normally, these two grouse seem to possess as many as six free 

 caudal vertebrae in addition to the pygostyle ; Bonasa um- 

 bel 1 u s may sometimes possess as many free ones ; otherwise the 

 rule is generally five. 



With all the galline characters apparent, the limb bones in the 

 representatives of the genus Tympanuchus are unusually stout and 

 inclined to be massive for a grouse. In the skeleton of the wing, 

 the humerus is the only bone that is pneumatic, while in the leg 

 pneumaticity is not only fully enjoyed by the femur, but apparently 

 also by the proximal third of the tibiotarsus. Most of the rest of the 

 skeleton is pneumatic in this genus, and the pneumatic scapula of 

 either side' exhibits a peculiar dilatation at its distal extremity. 

 Both os furcula and the coracoids are essentially galline, and pre- 

 sent but few distinctive characters. A coracoid as compared with 

 that bone in G . b a n k i v a shows its sternal end to be more 

 expanded, and the processes there seen, rather more conspicuous. 



