134 STRUCTURE AND CLASSIFICATION OF BIRDS 



rated ; but the perforation is reduced in size in Geococcyx, 

 Tinamus, and Hesperornis. 



Hind Limb. The hind limb of birds consists of femur, 

 tibia and fibula, tarsus, metatarsus, and phalanges. 



In all birds the femur is shorter than the tibia, the pro- 

 portions varying much. It seerns impossible to place those 

 birds in which the difference is least at the base of the series, 

 on account, of course, of a resemblance so far to reptiles, 

 since relative importance of the fore and hind limb appears 

 to have something to do with the matter. From FUE- 

 BEINGEE'S tables ' it is to be gathered that Fregata is the 

 bird in which the difference between those two segments of 

 the leg is least. It is most pronounced in the divers, flamingo, 

 and Tubinares. A bone of some classificatory importance is 

 the patella, 2 a sesamoid on the upper surface of the knee. 

 This bone is not ossified at all in Colymbns, but is enormous 

 in the grebes and in Hesperornis, in which latter, as in 

 Phalacrocorax and Biziura lobata, it is perforated by the 

 tendon of the ambiens. In Plotus the patella is grooved only 

 for this tendon. 



In no bird (except as an occasional abnormality) is the 

 fibula a complete bone. It fails below, and does not reach 

 the tarsus. It is usually more or less coalescent with the 

 tibia. The latter is a strong bone with a crest in front, which 

 is enormous in the divers and Hesperornis. The distal end 

 of the tibia is formed by a portion of the tarsus, of which 

 the remaining portion is coalesced with the metatarsus. The 

 tarsus in the embryo 3 consists of three chondrites, a tibiale, 

 a fibulare, and a distale. The latter represents the separate 

 distal elements of the tarsus fused. The tibiale sends up- 



1 KESSLER (' Osteologie der Vogelfiisse,' Bull. Soc. Nat. Mosc. 1841) has also 

 given tables. The value of the ' long bones ' of the leg for ' defining orders, 

 families, and often genera ' is plainly set forth in this paper. See also MILNE- 

 EDWABDS, Oiscaux Fossilcs de. la France, where further information is to be found. 



2 J. KACZANDEK, ' Beitrag zur Lehre iiber die Entwicklungsgeschichte der 

 Patella,' Mt. Embr. lust. Wien, (2) ii. 1887, p. 12. 



3 G. BAUK, ' Der Tarsus der Vogel u. Dinosaurier,' Morpli. J.B. viii. 1883, 

 p. 417 ; E. G. MOKSE, ' On the Carpus and Tarsus of Birds,' Ann. Lye. Neui 

 York, x. 1873, p. 141, and ' On the Identity of the Ascending Process of the 

 Astragalus,' &c., Anniv. Mem. Bost. Soc. Nat. Hist. 1880. 



