342 BTTLLETIN UNITED STATES GEOLOGICAL SUEVEY. [Vol. VL 



in a fine specimen of Tetrao ohscurus, we find the pre-acetabular por- 

 tions of the ilia very much depressed below the general surface of the 

 bone, and quite concave. The convex anterior borders are embellished 

 with a flattened rim that bounds them, often produced forwards as two 

 median sharp-pointed processes. The post-acetabular surface is raised, 

 and including the sacrum forms a gently convex surface ; the " gluteal 

 ridges" dividing these two regions commence with the anterior iliac 

 borders and are conveyed clear round to form the posterior ones of the 

 same bones, describing two great Ss, the lower and outer curves of 

 which pass by the acetabular projections, points where in some birds 

 they terminate. 



Among the principal features to be noted in a lateral view is that the 

 shaft of the pubis is in nearly all instances free from the ischium after 

 quitting the cotylis and forming the elliptical obdurator foramen. If it 

 anchyloses with this bone at all it usually- occurs just behind that orifice. 



The ischium overlaps the pubis at two points — one quite broadly near 

 its middle, and, again, by a process at its outer and inferior angle. 



The ^'ischiatic foramen" is the largest vacuity of the group of three 

 that here present themselves; its boundary is sub -elliptical, with its 

 major axis depressed posteriorly if it were produced. 



The cotyloid cavity, or ring rather, is markedly circular, both its in- 

 ternal and external margins, the former of which is not a little smaller, 

 thus affording a very good and quite extensive surface for the head of 

 the femur ; the anti-trochanterian process or facet directed backwards 

 IS likewise ample, so that the femora are well supplied with articulating 

 surfaces. 



There seems to be among the Grouse a predisposition for the ilia" to 

 overhang the region of the ischiadic foramina ; it is most successfidly 

 carried out in Cupidonia. Viewed from below, we are struck with the 

 amount of room and space these combined bones inclose ; the profundity 

 of the pelvic basin. This is very much enhanced by broad reduplicatures 

 of the ilia and ischia behind, and a general though even constriction of 

 a prominent rounded border or rather ridge that extends from the fourth 

 diapophysial abutment of the vertebra against the ilia on either side to 

 the outer angles of the ischia. Within, too, we often find about and at the 

 base of these iliac fossae apertures for the entrance of air into these bones; 

 such pneumatic foramina are also seen beyond the os pubis and below 

 the cotylis on either aspect. 



The pubic extremities never meet behind, though in many species they 

 are very long and usually take the curve given them by the ischia just 

 before leaving these bones. Their distal extremities are flattened in Cen- 

 trocercns and generally more expanded than among the other varieties. 



The Y)e\\is of Cupidonia is so different from the general description 

 we have just given that the author felt justified in giving to his reader 

 two additional figures that present superior and lateral views of the bone 

 in that bird ; in comparing it with other figures given one cannot avoid 



