No. 2. J SHUFELDT ON NORTH AMERICAN TETRAONID^. 345 



oval; the fibular ridge of tbe upper and outer third of this portion of 

 the bone in Centrocercus is about 2 centimetres long, and appears to be 

 little more than a raised and roughened line, the merest apologj^ for an 

 invitation to its delicate comiianiou, the fibula, to ever form a lasting 

 and inseparable bond of union with it, a contract which this free and 

 independent little bone has never entered into, in any of the Grouse or 

 the Odontophorincv, notwithstanding the fact that they are destined to 

 be the closest of associates their whole life through. Below the fibular 

 ridge Ave find the nutrient foramen, but otherwise the tibial shaft is very 

 straight and almost entirely devoid of any markings, at least to that point 

 anteriorly, where the ascending groove coming from between the con- 

 dyles impresses it, and that, with an increasing intensity to its termi- 

 nation over the intercoodyloid notch ; at this point a bony bridge is 

 thrown across obliquely, the outer abutment of which is the lower (Plate 

 IX. Fig. GO). 



The most engaging points of interest so far as the tibia is concerned, 

 (as many a young Grouse that long since has yielded up his life to the 

 cause of ornithotomy can testify to) centre about the distal extremity of 

 the bone. After careful examinations of the recentlj-killed subjects, 

 dried skeletons, and carmine-stained specimens, the following results 

 seem to be presented with greater or less distinctness in every case, 

 and these results correspond verj'^ nearly with Professor Morse's invalu- 

 able investigations and studies : In the young of Centrocercus, several 

 days after leaving the nest, we observe at the future site of the tibial 

 condyles, encased in the then articular cartilage, on either side, a free 

 reniform ossicle. These ossif}' to the surface in time, and the outer 

 becomes the Jibularc, the inner the tibiale ; both are tarsal bones. It 

 will take time and further research to decide definitely as to which is 

 the OS calcis, and which the astragalus. Above the former, and a little 

 towards a mid-shaft position, there is yet another free ossicle ; it is the 

 intermedium of Morse. That it is another tarsal bone there can be no 

 doubt, we believe, at this date ; but before we decide upon its special 

 homology we must satisfy ourselves by further research and dissection 

 of the young at various stages of development in birds and other ver- 

 tebrates where this bone constitutes a feature of the skeleton. 



It soon fuses with the tibiale, and the latter with the fibulare, so that 

 the faintest traces are alone discernible in the bird at two mouths of 

 age (Plate IX, Fig. G7, E'), at which stage tlie subsequent bony span for 

 tendinous confinement above them has not commenced to ossify. 



In the adult the apex of the intermedium affords attachment to the 

 oblique ligament that is attached higher up on the inner aspect of the 

 shaft, that also holds some of the extensor tendons in position. The 

 condyles in mature birds have an antero-posterior position at the 

 extremity of the tibia ; these are of a uniform outline, and the inter-con- 

 dyloid notch that separates them can boast of no particular depth until 

 it arrives on the anterior aspect of the bone. Externally and laterally 



