346 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Vol.\l. 



almost within the limits of the outline of the outer condyle we find two 

 tubercles, one above another ; the lower is for ligamentous attachment, 

 the upper is the remnant and only existing evidence of the lower extrem- 

 ity of the fibula. A similar tubercle is found on the opposite side cor- 

 responding to the lower one just described on the outer aspect. 



The fibula is freely detached and never completely anchyloses with 

 the tibia. Its proximal extremity is clubbed, enlarging very much as it 

 rises above the condylar surface of its companion from the fibular ridge ; 

 it is laterally comi^ressed and convex above at the summit. In many 

 Grouse the attenuated remains of its extension below can be traced on 

 the shaft of the tibia, which bone has nearly absorbed this third of its 

 weaker associate. 



In my papers upon the osteology of Spheotyto and EremopMla the old 

 term of the "calcaneal" process was still retained for that prominent 

 projection found at the superior and hinder end of the bone tarso-meta- 

 tarsus. The older comparative anatomists gave it this name, probably, 

 in view of the fact that this apophysis might eventually be proved to be 

 the OS calcis, but such advances have been instituted in the study of the 

 avian tarsus, that we may say that this process is not in any way enti- 

 tled to the term ; it does not even come in contact with the primoidal 

 element of that tarsal segment, so this apiiellation will here be aban- 

 doned, and as far as we are concerned such an error will receive no fur- 

 ther encourageuient in the way of ornithotomical recordation. 



In the early life of the chick of the Grouse we have been discussing, 

 the combined tarsals are surmounted by a third plate of cartilage, that 

 subsequently ossifies, apparently by one centre. The bone thus formed, 

 the centrale, we believe undoubtedly to rej3reseut either a single tarsal 

 element or the connate bones of the second row. 



At this age the metatarsals that combine to form the shaft of the 

 tarso-metatarsus are still easily individualized, though well on the road 

 toward permanent fusion. It will be observed that we still retain the 

 term tarso-metatarsus, and we think justly so, as the compound bone of 

 the mature bird has combined with it at least one of the tarsal bones. 

 The tibia could with equal reason be termed the tibio-tarsus, and again 

 the compound bone in menus, the carpo metacarpus, but for obvious 

 reasons such innovations are not always advisable. 



We discover in Centrocercus and Tetrao canadensis — in that strong 

 inelastic cartilage that is found at the back of the tarsal joint in all the 

 Grouse, on the inner side — a concavo-convex free bone, nearly a centi- 

 metre long, in the Sage Cock, and two or three millimetres wide ; this 

 ossicle must be regarded only as a sesamoid, though it is nearly as large 

 as the patella, and in no way as constituting one of the tarsal bones. 



The posterior process, or the tendinous process (the "calcaneal" of the 

 older authors), at the head of the combined metatarsals, is both verti- 

 cally grooved and i)erforated for the passage of tendons ; from its inner 

 and posterior angle in many of the Tetraonince it sends down a thin plate 



