No. 2.] SnUFELDT ON NORTH AMERICAN TETRAONID.E. 341 



meet a certain requisite need, the bony dorsal vault to shield the gen- 

 erative and urinary apparatuses and bear the pelvic limbs. This arch 

 is so inclined that the remaining vertebra? soon, by their outgrowing 

 apophyses, come in contact with it, ultimately fusing to develop the 

 compound bone, usually termed sacrum. A good idea may be gained 

 of the form of the ischium and os pubis by an inspection of Fig. 62, 

 Plate VIII, at this stage of the bird's existence. For very obvious 

 reasons the stoutest part of the ischiatic element surrounds the acetab- 

 ulum; an inferior process, extending towards a similar one directed 

 backwards by the ilium, grasps the head of the os pubis between them. 

 From this point the ischium sends backwards a triangular thin plate, 

 rounded behind, that is intermediate in length between ilium and 

 pubis. This latter is satisfied to follow suit and direction by contribut- 

 ing a simple and diminutive spine. This crosses soon after the pubis 

 has given its share to the cotylis, the obduratic notch of the ischium, 

 converting, as the bones fuse, it into the obdurator foramen. 



We give an additional figure in the Plate, 63, that exhibits the further 

 advances of these elements towards maturity. The bird is now nearly 

 two months old, and it will be seen that one after another the vertebrae 

 impinge upon the iliac margins within. The ilia and ischia extend be- 

 hind, including vertebra after vertebra, from a series that at this age 

 might be easily taken for an extraordinary number .of coccygeal seg- 

 ments. The head of the pubis at this time is entirely out of proportion 

 with its rib-like extension, it having so spread and insinuated itself into 

 the formation of the cotyloid ring as to form aboujt one-fifth of its cir- 

 cumference. At this stage, too, the necessary cartilage begins to be 

 thrown out to form the future anti-trochanter on the iliu'm. 



So much for i^elvic development ; now let us examine this bony basin 

 as it is seen in the full-grown representatives of the genera. 



The manner in which the confluent sacral vertebrae meet the ossa in- 

 nomiuata forming the ilio-neural canals and sacral sutures has already 

 been defined above, so that here nothing more need be said of the mid- 

 section of the pelvis taken as a whole. We give among the plates fig- 

 ures of the pelves of Centrocercus, Tetrao canadensis., Lagopus, and Cu- 

 pidonia from the skeletons of the adult birds, and if the reader will 

 compare the lateral views given of the Ptarmigan and the Sage Cock 

 he will see how few the diiferences are that exist between the two birds, 

 and the same may be said, we assure you, for its superior and inferior 

 views; in fact in the case of Lagopus it may be stated that its pehis in 

 most instances is the perfect miniature of this bone found in Centrocercus 

 in all respects. This applies, also, though not quite as strictly, to 

 Tetrao and Bonasa. Of course certain minor differences are easily to 

 be picked out, such as a greater fulness there, as slightly deeper depres- 

 sion here, and so on ; still our plate of Tetrao canadensis represents the 

 general pattern of the pelvis among all the Xorth American Grouse, save 

 Pedicecetes and Cupidonia. In the superior aspect of this confluent bone 



