On the Tarsus and Carjms of Birds. 145 



sentiug this process. The fibulare unites with the tihiale at 

 a very early age, there being a sort of hour-ghiss shaped 

 constriction between them. From their resemblance, at this 

 stage, to a similar bone in Laelaps, as determined by Prof. 

 Cope, the correctness of the term astragalo-calcaneum, 

 applied to that bone by him, is confirmed. 



The coalescence of these two bones, forms the peculiar 

 bi-condylar trochlea of the distal end of a bird's tibia ; a 

 firm joint is thus rendered ; for the inner condyloid margins, 

 thus produced, bear against either side of the centrale, which 

 early unites with the proximal ends of the metatarsals. 



The mid-metatarsal is generally the shortest at the proxi- 

 mal end, and the centrale fills up the depression thus 

 made. 



In the crow blackbird the proximal end of the mid-meta- 

 tarsal at an early stage is crowded back by the metatarsals 

 upon each side of it, as is usual in adult birds.* In the 

 same bird the centrale is small and round, and imites chiefly 

 with the mid-metatarsal. In the spotted sandpiper, the 

 centrale is lozenge-shaped, and caps the three metatarsals. 

 The tihiale and fibulare unite, forming a symmetrical hour- 

 glass shaped bone. 



In the bank swallow and kingbird, the centrale is similar 

 in shape to that of the sandpiper, and in like manner caps 

 the metatarsals. In the bluebird, the centrale is very large 

 and irregular in shape, and unites first with the second meta- 

 tarsal, but overlaps the others. In the yellow warbler the 

 centrale is very large and irregular in shape, presenting two 

 conspicuous prominences upon its articular face. In the 

 eave swallow, the centrale unites with the metatarsus a long 

 time before the tihiale and fibulare have united with each 

 other, or with the tibia. 



In all the birds examined, the tihiale and fibulare, with 

 one exception, anchylose together before they unite with the 



*See Owen, immature Dinornis crassus. Trans. Zool. Soc, VI, pi. 6, and Dr. R. O. 

 Cuuuiugham's Rhea Darivinii, Proc. Zool. Soc, 1871, pi. 6. figs. 7 and 8. 



